Nanofiber‐Based Bulk‐Heterojunction Organic Solar Cells Using Coaxial Electrospinning

Nanofibers consisting of the bulk heterojunction organic photovoltaic (BHJ–OPV) electron donor–electron acceptor pair poly(3‐hexylthiophene):phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) are produced through a coaxial electrospinning process. While P3HT:PCBM blends are not directly electrospinnable, P3HT:PCBM‐containing fibers are produced in a coaxial fashion by utilizing polycaprolactone (PCL) as an electrospinnable sheath material. Pure P3HT:PCBM fibers are easily obtained after electrospinning by selectively removing the PCL sheath with cyclopentanone (average diameter 120 ± 30 nm). These fibers are then incorporated into the active layer of a BHJ–OPV device, which results in improved short‐circuit current densities, fill factors, and power‐conversion efficiencies (PCE) as compared to thin‐film devices of identical chemical composition. The best‐performing fiber‐based devices exhibit a PCE of 4.0%, while the best thin‐film devices have a PCE of 3.2%. This increase in device performance is attributed to the increased in‐plane alignment of P3HT polymer chains on the nanoscale, caused by the electrospun fibers, which leads to increased optical absorption and subsequent exciton generation. This methodology for improving device performance of BHJ–OPVs could also be implemented for other electron donor–electron acceptor systems, as nanofiber formation is largely independent of the PV material.     

Comparison of the Morphology Development of Polymer–Fullerene and Polymer–Polymer Solar Cells during Solution‐Shearing Blade Coating

In this work, the detailed morphology studies of polymer poly(3‐hexylthiophene‐2,5‐diyl) (P3HT):fullerene(PCBM) and polymer(P3HT):polymer naphthalene diimide thiophene (PNDIT) solar cell are presented to understand the challenge for getting high performance all‐polymer solar cells. The in situ X‐ray scattering and optical interferometry and ex situ hard and soft X‐ray scattering and imaging techniques are used to characterize the bulk heterojunction (BHJ) ink during drying and in dried state. The crystallization of P3HT polymers in P3HT:PCBM bulk heterojunction shows very different behavior compared to that of P3HT:PNDIT BHJ due to different mobilities of P3HT in the donor:acceptor glass. Supplemented by the ex situ grazing incidence X‐ray diffraction and soft X‐ray scattering, PNDIT has a lower tendency to form a mixed phase with P3HT than PCBM, which may be the key to inhibit the donor polymer crystallization process, thus creating preferred small phase separation between the donor and acceptor polymer.     

Intra‐Molecular Donor–Acceptor Interaction Effects on Charge Dissociation,Charge Transport,and Charge Collection in Bulk‐Heterojunction Organic Solar Cells

This article reports experimental studies on internal charge dissociation, transport, and collection by using magnetic field effects of photocurrent (MFE_PC) and light‐assisted dielectric response (LADR) in highly‐efficient organic solar cells based on photovoltaic polymer PTB2 and PTB4 with intra‐molecular “donor–acceptor” interaction. The MFE_PC at low‐field (< 150 mT) indicates that intra‐molecular “donor‐acceptor” interaction generates charge dissociation in un‐doped PTB2 and PTB4 films, which is similar to that in lightly doped P3HT (Poly(3‐hexylthiophene)) with 5 wt% PCBM (1‐(3‐methyloxycarbonyl)‐propyl‐1‐phenyl (6,6) C₆₁). After PTB2 and PTB4 are mixed with PCBM to form bulk‐heterojunctions, the MFE_PC at high‐field (> 150 mT) reveals that the charge‐transfer complexes formed at PTB2:PCBM and PTB4:PCBM interfaces have much lower binding energies due to stronger electron‐withdrawing abilities, as compared to the P3HT:PCBM device, towards the generation of photocurrent. Furthermore, the light‐assisted dielectric response: LADR indicates that the PTB2:PCBM and PTB4:PCBM solar cells exhibit larger capacitances relative to P3HT:PCBM device under photoexcitation. This reflects that the PTB2:PCBM and PTB4:PCBM bulk heterojunctions have more effective charge transport and collection than the P3HT:PCBM counterpart. As a result, our experimental results indicate that intra‐molecular “donor‐acceptor” interaction plays an important role to enhance charge dissociation, transport, and collection in bulk‐heterojunction organic solar cells.     

Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge‐Transfer State Electroluminescence

Charge‐transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free‐carrier bimolecular recombination at the donor‐acceptor interface, rather than a charge‐trap‐mediated process. The fingerprint of the presence of nonradiative trap‐assisted recombination, a voltage‐dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination‐intensity dependence of the open‐circuit voltage.     

High‐Efficiency CsPbI2Br Perovskite Solar Cells with Dopant‐Free Poly(3‐hexylthiophene) Hole Transporting Layers

CsPbI₂Br is emerging as a promising all‐inorganic material for perovskite solar cells (PSCs) due to its more stable lattice structure and moisture resistance compared to CsPbI₃, although its device performance is still much behind this counterpart. Herein, a preannealing process is developed and systematically investigated to achieve high‐quality CsPbI₂Br films by regulating the nucleation and crystallization of perovskite. The preannealing temperature and time are specifically optimized for a dopant‐free poly(3‐hexylthiophene) (P3HT)‐based device to target dopant‐induced drastic performance degradation for spiro‐OMeTAD‐based devices. The resulting P3HT‐based device exhibits comparable power conversion efficiency (PCE) to spiro‐OMeTAD‐based devices but much enhanced ambient stability with over 95% PCE after 1300 h. A diphenylamine derivative is introduced as a buffer layer to improve the energy‐level mismatch between CsPbI₂Br and P3HT. A record‐high PCE of 15.50% for dopant‐free P3HT‐based CsPbI₂Br PSCs is achieved by alleviating the open‐circuit voltage loss with the buffer layer. These results demonstrate that the preannealing processing together with a suitable buffer layer are applicable strategies for developing dopant‐free P3HT PSCs with high efficiency and stability.     

Selective Dye Loading at the Heterojunction in Polymer/Fullerene Solar Cells

Selective dye loading at the polymer/fullerene interface was studied for ternary blend bulk heterojunction solar cells, consisting of regioregular poly(3‐hexylthiophene) (RR‐P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light‐harvesting dye. The photocurrent density and power conversion efficiency of the ternary blend solar cells were most improved by loading SiPc with a content of 4.8 wt%. The absorption and surface energy measurements suggested that SiPc is located in the disordered P3HT domains at the RR‐P3HT/PCBM interface rather than in the PCBM and crystal P3HT domains. From the peak wavelength of SiPc absorption, the local concentration of SiPc ([SiPc]_Local) was estimated for the RR‐P3HT:PCBM:SiPc ternary blends. Even for amorphous films of regiorandom P3HT (RRa‐P3HT) blended with PCBM and SiPc, [SiPc]_Local was higher than the original content, suggesting dye segregation into the RRa‐P3HT/PCBM interface. For RR‐P3HT:PCBM:SiPc blends, [SiPc]_Local increased with the increase in the P3HT crystallinity. Such interfacial segregation of dye molecules in ternary blend films can be rationally explained in terms of the surface energy of each component and the crystallization of P3HT being enhanced by annealing. Notably, the solvent annealing effectively segregated dye molecules into the interface without the formation of PCBM clusters.     

Study of Burn‐In Loss in Green Solvent‐Processed Ternary Blended Organic Photovoltaics Derived from UV‐Crosslinkable Semiconducting Polymers and Nonfullerene Acceptors

This work deals with the investigation of burn‐in loss in ternary blended organic photovoltaics (OPVs) prepared from a UV‐crosslinkable semiconducting polymer (P2FBTT‐Br) and a nonfullerene acceptor (IEICO‐4F) via a green solvent process. The synthesized P2FBTT‐Br can be crosslinked by UV irradiation for 150 s and dissolved in 2‐methylanisole due to its asymmetric structure. In OPV performance and burn‐in loss tests performed at 75 °C or AM 1.5G Sun illumination for 90 h, UV‐crosslinked devices with PC₇₁BM show 9.2% power conversion efficiency (PCE) and better stability against burn‐in loss than pristine devices. The frozen morphology resulting from the crosslinking prevents lateral crystallization and aggregation related to morphological degradation. When IEICO‐4F is introduced in place of a fullerene‐based acceptor, the burn‐in loss due to thermal aging and light soaking is dramatically suppressed because of the frozen morphology and high miscibility of the nonfullerene acceptor (18.7% → 90.8% after 90 h at 75 °C and 37.9% → 77.5% after 90 h at AM 1.5G). The resulting crosslinked device shows 9.4% PCE (9.8% in chlorobenzene), which is the highest value reported to date for crosslinked active materials, in the first green processing approach.     

The Physics of Small Molecule Acceptors for Efficient and Stable Bulk Heterojunction Solar Cells

Organic bulk heterojunction solar cells based on small molecule acceptors have recently seen a rapid rise in the power conversion efficiency with values exceeding 13%. This impressive achievement has been obtained by simultaneous reduction of voltage and charge recombination losses within this class of materials as compared to fullerene‐based solar cells. In this contribution, the authors review the current understanding of the relevant photophysical processes in highly efficient nonfullerene acceptor (NFA) small molecules. Charge generation, recombination, and charge transport is discussed in comparison to fullerene‐based composites. Finally, the authors review the superior light and thermal stability of nonfullerene small molecule acceptor based solar cells, and highlight the importance of NFA‐based composites that enable devices without early performance loss, thus resembling so‐called burn‐in free devices.     

Influence of Phonon Scattering on Exciton and Charge Diffusion in Polymer‐Fullerene Solar Cells

Thermally activated transport and phonon scattering in P3HT:PCBM (poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenylC61‐butyric acid methyl ester) bulk heterojunction (BHJ) organic solar cells is studied via temperature‐dependent external‐quantum‐efficiency (EQE) spectroscopy. The hopping barriers for combined exciton and charge transport are balanced for the individual blended materials in a sample, which possesses a blending ratio and a morphology that give rise to a maximal power‐conversion efficiency. Increasing the PCBM weight fraction leads to a reduction of exciton hopping barriers in PCBM, while for P3HT exciton hopping barriers remain constant. This reduction of PCBM exciton hopping barriers is attributed to a higher PCBM crystallinity in the PCBM‐rich solar cell as compared to the BHJ with the optimized blending ratio. The morphology‐dependent difference in exciton hopping activation energies between P3HT and PCBM is attributed to a higher impact of phonon scattering in P3HT than in PCBM, as concluded from the much stronger decrease of P3HT‐related temperature‐dependent external quantum efficiencies above room temperature in the PCBM‐rich BHJ solar cell. All EQE data of P3HT:PCBM‐based BHJ solar cells is modeled consistently over a broad temperature range by a simple analytical expression involving temperature activation and phonon scattering, without the need to distinguish two separate hopping regimes.     

Non‐Fullerene Acceptor‐Based Bulk Heterojunction Polymer Solar Cells: Engineering the Nanomorphology via Processing Additives

The performance of bulk heterojunction solar cells made from blends of a non‐fullerene acceptor, N,N′‐bis(2‐ethylhexyl)‐2,6‐bis(5″‐hexyl‐[2,2′;5′,2″]terthiophen‐5yl)‐1,4,5,8‐naphthalene diimide (NDI‐3TH), and poly(3‐hexylthiophene) (P3HT) donor is enhanced 10‐fold by using a processing additive in conjunction with an electron‐blocking and a hole‐blocking buffer layers. The power conversion efficiency of P3HT:NDI‐3TH solar cells improves from 0.14% to 1.5% by using a processing additive (1,8‐diiodooctane) at an optimum concentration of 0.2 vol%, which is far below the 2‐3 vol% optimum concentrations found in polymer/fullerene systems. TEM and AFM imaging show that the size and connectivity of the NDI‐3TH domains in the phase‐separated P3HT:NDI‐3TH blends vary strongly with the concentration of the processing additive. These results demonstrate, for the first time, that processing additives can be effective in the optimization of the morphology and performance of bulk heterojunction polymer solar cells based on non‐fullerene acceptors.     

The Importance of Fullerene Percolation in the Mixed Regions of Polymer–Fullerene Bulk Heterojunction Solar Cells

Most optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ～80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.     

Efficiency Enhancement of Polymer Solar Cells Based on Poly(3‐hexylthiophene)/Indene‐C70 Bisadduct via Methylthiophene Additive

Photovoltaic performance of polymer solar cells based on poly(3‐hexylthiophene) (P3HT) as the donor and indene‐C₇₀ bisadduct (IC₇₀BA) as the acceptor is improved by adding 3 vol% 3‐methylthiophene (MT) or 3‐hexylthiophene (HT) as processing additives. The results of UV‐vis absorption spectroscopy, X‐ray diffraction analysis and atomic force microscopy indicate that with the MT or HT processing additive, the active layer of the blend of P3HT/IC₇₀BA showed strengthened absorbance, enhanced crystallinity and improved film morphology. The power conversion efficiency (PCE) of the PSCs was improved from 5.80% for the device without the additive to 6.35% for the device with HT additive and to 6.69% with MT additive. The PCE of 6.69% is the top value reported so far for the PSCs based on P3HT.     

Polymer:Nonfullerene Bulk Heterojunction Solar Cells with Exceptionally Low Recombination Rates

Organic semiconductors are in general known to have an inherently lower charge carrier mobility compared to their inorganic counterparts. Bimolecular recombination of holes and electrons is an important loss mechanism and can often be described by the Langevin recombination model. Here, the device physics of bulk heterojunction solar cells based on a nonfullerene acceptor (IDTBR) in combination with poly(3‐hexylthiophene) (P3HT) are elucidated, showing an unprecedentedly low bimolecular recombination rate. The high fill factor observed (above 65%) is attributed to non‐Langevin behavior with a Langevin prefactor (β/β_L) of 1.9 × 10^?4. The absence of parasitic recombination and high charge carrier lifetimes in P3HT:IDTBR solar cells inform an almost ideal bimolecular recombination behavior. This exceptional recombination behavior is explored to fabricate devices with layer thicknesses up to 450 nm without significant performance losses. The determination of the photoexcited carrier mobility by time‐of‐flight measurements reveals a long‐lived and nonthermalized carrier transport as the origin for the exceptional transport physics. The crystalline microstructure arrangement of both components is suggested to be decisive for this slow recombination dynamics. Further, the thickness‐independent power conversion efficiency is of utmost technological relevance for upscaling production and reiterates the importance of understanding material design in the context of low bimolecular recombination.     

Overcoming Interfacial Losses in Solution‐Processed Organic Multi‐Junction Solar Cells

Organic solar cells are promising in terms of full‐solution‐processing which enables low‐cost and large‐scale fabrication. While single‐junction solar cells have seen a boost in power conversion efficiency (PCE), multi‐junction solar cells are promising to further enhance the PCE. In all‐solution‐processed multi‐junction solar cells, interfacial losses are often encountered between hole‐transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high‐performance donor and acceptor materials in multi‐junction solar cells. Here, the authors report on a systematic study of interface losses in both single‐junction and multi‐junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time‐of‐flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO _x nanoparticles successfully overcomes the interfacial losses in both single‐ and multi‐junction solar cells based on various active layers by reducing interface protonation, promoting better energy‐level alignment, and forming a dense and smooth layer. Solution‐processed single‐junction solar cells are demonstrated to reach the same performance as with evaporated MoO _x (over 7%). Multi‐junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO _x nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO _x nanoparticles show FF smaller than 50% and PCE less than 5%.     

A Thieno[3,2‐c]Isoquinolin‐5(4H)‐One Building Block for Efficient Thick‐Film Solar Cells

An aromatic lactam acceptor unit, thieno[3,2‐c]isoquinolin‐5(4H)‐one (TIQ), is developed. Compared with its analogues, dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one (DTP) and phenanthridin‐6(5H)‐one (PN), TIQ shows its advantage in constructing donor–acceptor (D–A) copolymers for efficient solar cells. TIQ‐based D–A copolymer PTIQ4TFBT delivers a power conversion efficiency (PCE) of 10.16% in polymer:fullerene solar cells, while those based on DTP and PN copolymers, PDTP4TFBT and PPN4TFBT, afford PCEs around 8.5%. The higher performance of PTIQ4TFBT:PC₇₁BM solar cells originates from enhanced short‐circuit current density (J_sc) and fill factor (FF), because of favorable morphology, less bimolecular recombination, and balanced charge transport in the active layer. Moreover, the performance for PTIQ4TFBT:PC₇₁BM solar cells is less sensitive to active layer thickness than PDTP4TFBT:PC₇₁BM and PPN4TFBT:PC₇₁BM solar cells. Over 8% PCEs can be obtained from PTIQ4TFBT:PC₇₁BM solar cells when the active layer thickness is over 500 nm.     

Morphology‐Limited Free Carrier Generation in Donor/Acceptor Polymer Blend Solar Cells Composed of Poly(3‐hexylthiophene) and Fluorene‐Based Copolymer

The charge generation and recombination dynamics in polymer/polymer blend solar cells composed of poly(3‐hexylthiophene) (P3HT, electron donor) and poly[2,7‐(9,9‐didodecylfluorene)‐alt‐5,5‐(4′,7′‐bis(2‐thienyl)‐2′,1′,3′‐benzothiadiazole)] (PF12TBT, electron acceptor) are studied by transient absorption measurements. In the unannealed blend film, charge carriers are efficiently generated from polymer excitons, but some of them recombine geminately. In the blend film annealed at 160 °C, on the other hand, the geminate recombination loss is suppressed and hence free carrier generation efficiency increases up to 74%. These findings suggest that P3HT and PF12TBT are intermixed within a few nanometers, resulting in impure PF12TBT and disordered P3HT domains. The geminate recombination is likely due to charge carriers generated on isolated polymer chains in the matrix of the other polymer and at the domain interface with disordered P3HT. The undesired charge loss by geminate recombination is reduced by both the purification of the PF12TBT‐rich domain and crystallization of the P3HT chains. These results show that efficient free carrier generation is not inherent to the polymer/fullerene domain interface, but is possible with polymer/polymer systems composed of crystalline donor and amorphous acceptor polymers, opening up a new potential method for the improvement of solar cell materials.     

Deep Energetic Trap States in Organic Photovoltaic Devices

The nature of energetic disorder in organic semiconductors is poorly understood. In photovoltaics, energetic disorder leads to reductions in the open circuit voltage and contributes to other loss processes. In this work, three independent optoelectronic methods were used to determine the long‐lived carrier populations in a high efficiency N‐alkylthieno[3,4‐c]pyrrole‐4,6‐dione (TPD) based polymer: fullerene solar cell. In the TPD co‐polymer, all methods indicate the presence of a long‐lived carrier population of ～ 10¹⁵ cm^?3 on timescales ≥ 100 μs. Additionally, the behavior of these photovoltaic devices under optical bias is consistent with deep energetic lying trap states. Comparative measurements were also performed on high efficiency poly‐3‐hexylthiophene (P3HT): fullerene solar cells; however a similar long‐lived carrier population was not observed. This observation is consistent with a higher acceptor concentration (doping) in P3HT than in the TPD‐based copolymer.     

Alkyl Chain Regiochemistry of Benzotriazole‐Based Donor Polymers Influencing Morphology and Performances of Non‐Fullerene Organic Solar Cells

The effects of alkyl chain regiochemistry on the properties of donor polymers and performances of non‐fullerene organic solar cells are investigated. Two donor polymers (PfBTAZ and PfBTAZS) are compared that have nearly identical chemical structures except for the regiochemistry of alkyl chains. The optical properties and crystallinity of two polymers are nearly identical yet the PfBTAZ:O‐IDTBR blend exhibits nearly double domain size compared to the blend based on PfBTAZS:O‐IDTBR. To reveal the origins of the very different domain size of two blends, the morphology of neat polymer films is characterized, and it is found that PfBTAZ tends to aggregate into much larger polymer fibers without the presence of O‐IDTBR. This indicates that it is not the polymer:O‐IDTBR interactions but the intrinsic aggregation properties of two polymers that determine the morphology features of neat and blend films. The stronger aggregation tendency of PfBTAZ could be explained by its more co‐planar geometry of the polymer backbone arising from the different alkyl chain regiochemistry. Combined with the similar trend observed in another set of donor polymers (PTFB‐P and PTFB‐PS), the results provide an important understanding of the structure–property relationships that could guide the development of donor polymers for non‐fullerene organic solar cells.     

Overcoming Space‐Charge Effect for Efficient Thick‐Film Non‐Fullerene Organic Solar Cells

Organic solar cells (OSCs) containing non‐fullerene acceptors have realized high power conversion efficiency (PCE) up to 14%. However, most of these high‐performance non‐fullerene OSCs have been reported with optimal active layer thickness of about 100 nm, mainly due to the low electron mobility (≈10^?4–10^?5 cm² V^?1 s^?1) of non‐fullerene acceptors, which are not suitable for roll‐to‐roll large‐scale processing. In this work, an efficient non‐fullerene OSC based on poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′′′‐diyl)] (PffBT4T‐2OD):EH‐IDTBR (consists of electron‐rich indaceno[1,2‐b:5,6‐b′]dithiophene as the central unit and an electron‐deficient 5,6‐benzo[c][1,2,5]thiadiazole unit flanked with rhodanine as the peripheral group) with thickness‐independent PCE (maintaining a PCE of 9.1% with an active layer thickness of 300 nm) is presented by optimizing device architectures to overcome the space‐charge effects. Optical modeling reveals that most of the incident light is absorbed near the transparent electrode side in thick‐film devices. The transport distance of electrons with lower mobility will therefore be shortened when using inverted device architecture, in which most of the excitons are generated close to the cathode side and therefore substantially reduces the accumulation of electrons in the device. As a result, an efficient thick‐film non‐fullerene OSC is realized. These results provide important guidelines for the development of more efficient thick‐film non‐fullerene OSCs.     

Light‐Induced Degradation of Polymer:Fullerene Photovoltaic Devices: An Intrinsic or Material‐Dependent Failure Mechanism?

Although degradation mechanisms in organic photovoltaic devices continue to receive increased attention, it is only recently that the initial light‐induced failure, or so‐called burn‐in effect, has been considered. Both prototypical polythiophene:fullerene and polycarbazole:fullerene systems exhibit an exponential performance loss of ≈40% upon 150 h of continuous solar illumination. While the decrease in both the short‐circuit current (J_SC) and open‐circuit voltage (V_OC) is the origin of performance loss in poly(3‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PC₆₀BM), in poly(N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)):[6,6]‐phenyl‐C71‐butyric acid methyl ester (PCDTBT:PC₇₀BM) the decline of the fill factor dominates. By systematic variation of the interface layers, active layer thickness, and acceptor in polythiophene:fullerene cells, the loss in J_SC is ascribed to a degradation in the bulk of the P3HT:PC₆₀BM, while the drop in V_OC is reversible and arises from charge trapping at the contact interfaces. By replacing the C₆₀ fullerene derivative with a C₇₀ derivative, or by modifying the electron transport layer, the J_SC or V_OC, respectively, are stabilized. These insights prove that the burn‐in process stems from multiple concurrent failure mechanisms. Comparing the ageing and recovery processes in P3HT and PCDTBT blends results in the conclusion that their interface failures differ in nature and that burn‐in is a material dependent, rather than an intrinsic, failure mechanism.