Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high‐temperature preparation of metal oxides as a charge transport layer in most state‐of‐the‐art PSCs has been becoming a big stumbling block for future low‐temperature and large‐scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low‐temperature‐processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all‐solution‐processed PSCs, we propose a novel n‐type sulfur‐containing small molecule hexaazatrinaphtho[2,3‐c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10^?2 cm² V^?1 s^?1 as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via S? I or S? Pb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n‐type organic small molecules for high‐performance PSCs.     

Higher Mobility and Carrier Lifetimes in Solution‐Processable Small‐Molecule Ternary Solar Cells with 11% Efficiency

Solution‐processed small molecule (SM) solar cells have the prospect to outperform their polymer‐fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk‐heterojunction (BHJ) charge carrier mobility (<10^‐4 cm² V^‐1 s^‐1) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making large‐scale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC₇₁BM can be used to achieve SM‐based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10^‐4 cm² V^‐1 s^‐1), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC₇₁BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC₇₁BM solar cells demonstrate long‐term shelf stability (dark) for >1000 h, in 55% humidity air environment.     

Modified PEDOT Layer Makes a 1.52 V Voc for Perovskite/PCBM Solar Cells

The work functions for charge transport layers in perovskite solar cells affect device performance significantly. In this work, the regular poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is modified by adding a polymer electrolyte PSS‐Na to improve its HTL function in perovskite solar cells. The modified PEDOT:PSS films (called m‐PEDOT:PSS) possess higher work function than the regular one. Its energy level matches the valence band of perovskite very well, leading to enhanced V_oc and PCE (power conversion efficiency). When CH₃NH₃PbI₃ is used as the light absorber, the cell with PEDOT:PSS HTL gives a V_oc of 0.96 V and a PCE of 12.35%, while the cell with m‐PEDOT:PSS layer gives a V_oc of 1.11 V and a PCE of 15.56%. Enhanced V_oc and PCE are also achieved when CH₃NH₃PbI₂Br or CH₃NH₃PbBr₃ is used as the light absorber. The m‐PEDOT:PSS/CH₃NH₃PbBr₃/PC₆₁BM solar cells demonstrate an outstanding V_oc of 1.52 V.     

A Smooth CH3NH3PbI3 Film via a New Approach for Forming the PbI2 Nanostructure Together with Strategically High CH3NH3I Concentration for High Efficient Planar‐Heterojunction Solar Cells

The photovoltaic performance of perovskite solar cells (PVSCs) is extremely dependent on the morphology and crystallization of the perovskite film, which is affected by the deposition method. In this work, a new approach is demonstrated for forming the PbI₂ nanostructure and the use of high CH₃NH₃I concentration which are adopted to form high‐quality (smooth and PbI₂ residue‐free) perovskite film with better photovoltaic performances. On the one hand, self‐assembled porous PbI₂ is formed by incorporating small amount of rationally chosen additives into the PbI₂ precursor solutions, which significantly facilitate the conversion of perovskite without any PbI₂ residue. On the other hand, by employing a relatively high CH₃NH₃I concentration, a firmly crystallized and uniform CH₃NH₃PbI₃ film is formed. As a result, a promising power conversion efficiency of 16.21% is achieved in planar‐heterojunction PVSCs. Furthermore, it is experimentally demonstrated that the PbI₂ residue in perovskite film has a negative effect on the long‐term stability of devices.     

Structural Characterization of a Composition Tolerant Bulk Heterojunction Blend

The ratio of the donor and acceptor components in bulk heterojunction (BHJ) organic solar cells is a key parameter for achieving optimal power conversion efficiency (PCE). However, it has been recently found that a few BHJ blends have compositional tolerance and achieve high performance in a wide range of donor to acceptor ratios. For instance, the X2 :PC₆₁BM system, where X2 is a molecular donor of intermediate dimensions, exhibits a PCE of 6.6%. Its PCE is relatively insensitive to the blend ratio over the range from 7:3 to 4:6. The effect of blend ratio of X2 /PC₆₁BM on morphology and device performance is therefore systematically investigated by using the structural characterization techniques of energy‐filtered transmission energy microscopy (EF‐TEM), resonant soft X‐ray scattering (R‐SoXS) and grazing incidence wide angle X‐ray scattering (GIWAXS). Changes in blend ratio do not lead to obvious differences in morphology, as revealed by R‐SoXS and EF‐TEM. Rather, there is a smooth evolution of a connected structure with decreasing domain spacing from 8:2 to 6:4 blend ratios. Domain spacing remains constant from 6:4 to 4:6 blend ratios, which suggests the presence of continuous phases with proper domain size that may provide access for charge carriers to reach their corresponding electrodes.     

How Photoinduced Crosslinking Under Operating Conditions Can Reduce PCDTBT‐Based Solar Cell Efficiency and then Stabilize It

Bulk heterojunction (BHJ) photovoltaic devices made of PCDTBT (poly[N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]) and PC₇₀BM ([6,6]‐phenyl‐C₇₀‐butyric acid methyl ester) are among the most efficient and stable devices studied so far. However, during a short regime called “burn‐in”, a significant decrease of power conversion efficiency was observed. A study of the photochemical mechanisms involved in the PCDTBT:PCBM active layer exposed to light in encapsulated systems is presented. It is found that the photochemical reactions resulting from the absorption of light by PCDTBT involve crosslinking between the 2,7 carbazole unit of PCDTBT and the fullerene unit of PCBM. Those reactions stabilize the BHJ by avoiding the formation of microsized PCBM crystals known to cause failure of BHJ solar cells. Using classical electron paramagnetic resonance spectroscopy (EPR) (without illumination), paramagnetic defects along the polymer chains have been detected. The kinetics of defects intensity show a burn‐in trend. The evolution of their relaxation times upon aging is in good agreement with a structural change (crosslinking) of the BHJ observed from the nanomechanical properties. Finally, light‐induced electron paramagnetic resonance (LEPR) measurements performed on aged samples revealed that electron transfer is not significantly affected upon aging, confirming thus the stabilization of the BHJ in solar cell operating conditions.     

Effect of Charge Recombination on the Fill Factor of Small Molecule Bulk Heterojunction Solar Cells

Solution‐processed organic BHJ solar cells based on 3,6‐bis[5‐(benzofuran‐2‐yl)thiophen‐2‐yl]‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione (DPP(TBFu)₂) or poly(3‐hexylthiophene) blended with [6,6]‐phenyl‐C₆₀₍₇₀₎ ‐butyric acid methyl ester (PC₆₀₍₇₀₎ BM) behave differently under various irradiation intensities. Small molecule‐based DPP(TBFu)₂:PC₆₀ BM solar cells show up to 5.2% power conversion efficiency and a high fill factor at low light intensity. At 100 mW cm^?2 illumination, the efficiency and fill factor decrease, resulting in stronger power losses. Impedance spectroscopy at various light intensities reveals that high charge recombination is the cause of the low fill factor in DPP(TBFu)₂:PC₆₀ BM.     

Overcoming Microstructural Limitations in Water Processed Organic Solar Cells by Engineering Customized Nanoparticulate Inks

The application of conjugated polymer and fullerene water‐based nanoparticles (NP) as ecofriendly inks for organic photovoltaics (OPVs) is reported. A low bandgap polymer diketopyrrolopyrrole–quinquethiophene (PDPP5T‐2) and the methanofullerene PC₇₁BM are processed into three types of nanoparticles: pristine fullerene NPs, pristine polymer NPs, and mixed polymer:fullerene NPs, allowing the formation of bulk heterojunction (BHJ) composites with different domain sizes. Mild thermal annealing is required to melt the nanospheres and enable the formation of interconnected pathways within mixed phases. This BHJ is accompanied by a shrinkage of film, whereas the more compact layers show enhanced mobility. Consistently reduced recombination and better performance are found for mixed NP, containing both, the polymer and the fullerene within a single NP. The optimized solar cell processed by ultrasmall NPs delivers a power conversion efficiency of about 3.4%. This is among the highest values reported for aqueous processed OPVs but still lacks performance compared to those being processed from halogenated solvents. Incomplete crystallization is identified as the main root for reduced efficiency. It is nevertheless believed that postprocessing does not cut attraction from printing aqueous organic NP inks as a trendsetting strategy for the reliable and ecofriendly production of organic solar cells.     

Unveiling the Low‐Temperature Pseudodegradation of Photovoltaic Performance in Planar Perovskite Solar Cell by Optoelectronic Observation

The time evolution of the current–voltage characteristic of planar heterojunction perovskite solar cell (PSC) is studied within an operating temperature range of 200–325 K. The photovoltaic (PV) performance of PSC is found to be influenced by five carrier transport pathways, which strongly depend on operating temperature and light illumination. At low temperature, a severe degradation of PV performance is presented but temporary. This is attributed to ion accumulation at the TiO₂/CH₃NH₃PbI₃ and hole transport material/CH₃NH₃PbI₃ interfacial regions, as an origin of screening effect of built‐in field, evidenced by the low external quantum efficiency (EQE). By light illumination at open‐circuit, a steady PV performance will be reached and the stabilization time increases with decreasing temperature. The recovery of PV performance is attributed to ion diffusion in CH₃NH₃PbI₃ layer in the absence of electric field. The EQE observations indicate that photogenerated carriers are separated and collected efficiently after a long time light illumination due to a reduction of the screening effect. At high temperature, because of the low ion density at interfacial regions, the PV performance shows a quick response to light. These findings may help understanding of the mechanism of temperature‐dependent photogenerated carrier transport in the PSC.     

Layer‐by‐Layer Solution‐Processed Low‐Bandgap Polymer‐PC61BM Solar Cells with High Efficiency

Polymer solar cells (PSCs) are fabricated without solvent additives using a low‐bandgap polymer, PBDTTT‐C‐T, as the donor and [6,6]‐phenyl‐C61‐butyric‐acid‐methyl‐ester (PC₆₁BM) as the acceptor. Donor‐acceptor blend and layer‐by‐layer (LL) solution process are used to form active layers. Relative to the blend devices, the LL devices exhibit stronger absorption, better vertical phase separation, higher hole and electron mobilities, and better charge extraction at correct electrodes. As a result, after thermal annealing the LL devices exhibit an average power conversion efficiency (PCE) of 6.86%, which is much higher than that of the blend devices (4.31%). The best PCE of the LL devices is 7.13%, which is the highest reported for LL processed PSCs and among the highest reported for PC₆₁BM‐based single‐junction PSCs.     

The Crucial Influence of Fullerene Phases on Photogeneration in Organic Bulk Heterojunction Solar Cells

The conjugated polymer, poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (pBTTT‐C₁₆), allows a systematic tuning of the blend morphology by varying the acceptor type and fraction, making it a well‐suited structural model for studying the fundamental processes in organic bulk heterojunction solar cells. To analyze the role of intercalated and pure fullerene domains on charge carrier photogeneration, time delayed collection field (TDCF) measurements and Fourier‐transform photocurrent spectroscopy (FTPS) are performed on pBTTT‐C₁₆:[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) solar cells with various stoichiometries. A weak influence of excess photon energy on photogeneration along with a photogeneration having a weaker field dependence at increasing fullerene loading is found. The findings are assigned to a dissociation via thermalized charge transfer (CT) states supported by an enhanced electron delocalization along spatially extended PC₆₁BM nanophases that form in addition to a bimolecular crystal (BMC) for PC₆₁BM rich blends. The highly efficient transfer of charge carriers from the BMC into the pure domains are studied further by TDCF measurements performed on non‐intercalated pBTTT‐C₁₆:bisPC₆₁BM blends. They reveal a field dependent charge generation similar to the 1:4 PC₆₁BM blend, demonstrating that the presence of pure acceptor phases is the major driving force for an efficient, field independent CT dissociation.     

Aerosol‐Jet‐Assisted Thin‐Film Growth of CH3NH3PbI3 Perovskites—A Means to Achieve High Quality,Defect‐Free Films for Efficient Solar Cells

A high level of automation is desirable to facilitate the lab‐to‐fab process transfer of the emerging perovskite‐based solar technology. Here, an automated aerosol‐jet printing technique is introduced for precisely controlling the thin‐film perovskite growth in a planar heterojunction p–i–n solar cell device structure. The roles of some of the user defined parameters from a computer‐aided design file are studied for the reproducible fabrication of pure CH₃NH₃PbI₃ thin films under near ambient conditions. Preliminary power conversion efficiencies up to 15.4% are achieved when such films are incorporated in a poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate‐perovskite‐phenyl‐C71‐butyric acid methyl ester type device format. It is further shown that the deposition of atomized materials in the form of a gaseous mist helps to form a highly uniform and PbI₂ residue‐free CH₃NH₃PbI₃ film and offers advantages over the conventional two‐step solution approach by avoiding the detrimental solid–liquid interface induced perovskite crystallization. Ultimately, by integrating full 3D motion control, the fabrication of perovskite layers directly on a 3D curved surface becomes possible. This work suggests that 3D automation with aerosol‐jet printing, once fully optimized, could form a universal platform for the lab‐to‐fab process transfer of solution‐based perovskite photovoltaics and steer development of new design strategies for numerous embedded structural power applications.     

Efficient Indium‐Doped TiOx Electron Transport Layers for High‐Performance Perovskite Solar Cells and Perovskite‐Silicon Tandems

In addition to a good perovskite light absorbing layer, the hole and electron transport layers play a crucial role in achieving high‐efficiency perovskite solar cells. Here, a simple, one‐step, solution‐based method is introduced for fabricating high quality indium‐doped titanium oxide electron transport layers. It is shown that indium‐doping improves both the conductivity of the transport layer and the band alignment at the ETL/perovskite interface compared to pure TiO₂, boosting the fill‐factor and voltage of perovskite cells. Using the optimized transport layers, a high steady‐state efficiency of 17.9% for CH₃NH₃PbI₃‐based cells and 19.3% for Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃‐based cells is demonstrated, corresponding to absolute efficiency gains of 4.4% and 1.2% respectively compared to TiO₂‐based control cells. In addition, a steady‐state efficiency of 16.6% for a semi‐transparent cell is reported and it is used to achieve a four‐terminal perovskite‐silicon tandem cell with a steady‐state efficiency of 24.5%.     

Self‐Organization of Polymer Additive,Poly(2‐vinylpyridine) via One‐Step Solution Processing to Enhance the Efficiency and Stability of Polymer Solar Cells

Interfaces between the photoactive layers and electrodes play critical roles in controlling the performance of optoelectronic devices. Herein, a novel nonconjugated polymer additive (nPA), poly(2‐vinylpyridine) (P2VP), is reported for modifying the interfaces between the bulk‐heterojunction (BHJ) and cathode/metal oxide (MO) layers. The P2VP nPA enables remarkably enhanced power conversion efficiencies (PCEs) and ambient stability in different types of polymer solar cells (PSCs). Importantly, interfacial engineering can be achieved during deposition of the P2VP nPA‐containing BHJ active layer via simple, one‐step solution processing. The P2VP nPA has much higher surface energy than the BHJ active components and stronger interaction with the surface of MO, which affords spontaneous vertical phase separation from the BHJ layer on the MO surface by one‐step solution processing. The self‐assembled P2VP layer substantially reduces the work function and surface defect density of MO, thereby minimizing the charge‐extraction barrier and increasing the PCEs of the PSCs significantly, i.e., PTB7‐Th:PC₇₁BM (10.53%→11.14%), PTB7:PC₇₁BM (7.37%→8.67%), and PTB7‐Th:P(NDI2HD‐T) all‐PSCs (5.52%→6.14%). In addition, the lifetimes of the PSCs are greatly improved by the use of the P2VP nPA.     

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short‐circuit currents (J_SC) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open‐circuit voltages (V_OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl‐C61‐butyric acid methyl ester (PC₆₁BM), that achieves a high V_OC (0.9 V) with very low energy losses (E_loss = 0.52 eV) from the energy of absorbed photons, a respectable J_SC (13 mA cm^?2), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field‐dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge‐carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC₆₁BM. These results hold promise that given the appropriate morphology, the J_SC, V_OC, and FF can all be improved, even with very low energetic offsets.     

Synergic Interface and Optical Engineering for High‐Performance Semitransparent Polymer Solar Cells

In this study the thickness of the PTB7‐Th:PC₇₁BM bulk heterojunction (BHJ) film and the PF3N‐2TNDI electron transport layer (ETL) is systematically tuned to achieve polymer solar cells (PSCs) with optimized power conversion efficiency (PCE) of over 9% when an ultrathin BHJ of 50 nm is used. Optical modeling suggests that the high PCE is attributed to the optical spacer effect from the ETL, which not only maximizes the optical field within the BHJ film but also facilitates the formation of a more homogeneously distributed charge generation profile across the BHJ film. Experimentally it is further proved that the extra photocurrent produced at the PTB7‐Th/PF3N‐2TNDI interface also contributes to the improved performance. Taking advantage of this high performance thin film device structure, one step further is taken to fabricate semitransparent PSCs (ST‐PSCs) by using an ultrathin transparent Ag cathode to replace the thick Ag mirror cathode, yielding a series of high performance ST‐PSCs with PCEs over 6% and average visible transmittance between 20% and 30%. These ST‐PSCs also possess remarkable transparency color perception and rendering properties, which are state‐of‐the‐art and fulfill the performance criteria for potential use as power‐generating windows in near future.     

The Effect of Fluorination in Manipulating the Nanomorphology in PTB7:PC71BM Bulk Heterojunction Systems

The best performing low bandgap copolymers PTB series to date which is based on thieno[3,4‐b]thiophene‐alt‐benzodithiophene units blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM), have been the focus of polymer‐based solar cells. Here, novel fluorinated polymers PTB7‐Fx (fluorine units coupled with submonomer thieno[3,4‐b]thiophene) with varied degree of fluorination are used as electron donor materials. The PTB7‐Fx:PC₇₁BM bulk heterojunction (BHJ) films spin‐coated from the host solvent chlorobenzene without and with solvent additive 1,8‐diiodooctane (DIO) and the corresponding solar cell devices are systematically investigated to address the morphology‐efficiency relationship. Self‐assembled BHJ morphology is already observed for as‐spun blend films. After adding the solvent additive DIO, the pronounced ordered structures are suppressed and better intermixed films with much smaller domain sizes result. Full fluorination of the third C‐atom of thienothiophene gives rise to the highest power conversion efficiency. As the absorption properties, film morphology and crystallinity remain similar for different degrees of fluorination, the main influence of the photovoltaic performance is ascribed to the different lowest unoccupied molecular orbital (LUMO) of each polymer instead of the film morphology. Thus the device performance can be efficiently improved by tuning the energy level of the polymer without necessarily changing either the film nanomorphology or crystallinity dramatically.     

Charge Carrier Dynamics in a Ternary Bulk Heterojunction System Consisting of P3HT,Fullerene, and a Low Bandgap Polymer

The photoresponse of P3HT:PC₆₁BM based organic solar cells can be enhanced by blending the bulk heterojunction with the low band gap polymer Si‐ PCPDTBT. Organic solar cells containing the resulting ternary blend as the photoactive layer deliver short circuit currents of up to 15.5 mA cm^?2. Morphological studies show modest phase separation without the perturbation of the crystallinity of the P3HT:PC₆₁BM matrix, in accordance with the measured acceptable fill factors. Picosecond time‐resolved pump‐probe spectroscopy reveals that the sensitization of P3HT:PC₆₁BM with Si‐PCPDTBT involves the transfer of photogenerated positive polarons from the low band gap polymer to P3HT within few hundreds of picoseconds. Intensity dependent experiments in combination with global fitting show that the charge transfer from Si‐PCPDTBT to P3HT competes with non‐geminate charge carrier recombination of the holes in the Si‐PCPDTBT phase with electrons in the PC₆₁BM phase, both processes being of diffusive nature. At excitation densities corresponding to steady state conditions under one sun, modelling predicts hole transfer efficiencies exceeding 90%, in accordance with IQE measurements. At higher pump intensities, bimolecular recombination suppresses the hole transfer process effectively.     

Impact of Fullerene Molecular Weight on P3HT:PCBM Microstructure Studied Using Organic Thin‐Film Transistors

The clustering and diffusion of C₇₁‐butyric acid methyl ester (PC₇₁BM) in poly(3‐hexylthiophene) (P3HT) has been studied using single layer blend and bilayer organic field‐effect transistors (OFETs) and by atomic force microscopy (AFM). P3HT:PC₇₁BM blend based OFETs were found to undergo phase‐segregation upon annealing, which was detectable as a fall in electron mobility with increasing annealing temperature. By employing carefully designed bilayer P3HT:PC₇₁BM OFETs, the diffusion‐properties of PC₇₁BM in P3HT could additionally be inferred from electron mobility measurements. It was found that the prerequisite annealing temperatures for detectable PC₇₁BM clustering and diffusion in P3HT was approximately 20 °C higher than for PC₆₁BM. The diffusion coefficient of PC₆₁BM in P3HT was found to be several times higher that that of PC₇₁BM. The present work provides unique insights into the diffusion process of fullerenes in conjugated polymers and could prove highly valuable for future materials development and device optimization.     

High‐Efficiency Polymer Solar Cells Achieved by Doping Plasmonic Metallic Nanoparticles into Dual Charge Selecting Interfacial Layers to Enhance Light Trapping

Significantly increased power conversion efficiency (PCE) of polymer solar cells (PSCs) is achieved by applying a plasmonic enhanced light trapping strategy to a low bandgap conjugated polymer, poly(indacenodithiophene‐ co‐phananthrene‐quinoxaline) (PIDT‐PhanQ) and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) based bulk‐heterojunction (BHJ) system. By doping both the rear and front charge‐selecting interfacial layers of the device with different sizes of Au NPs, the PCE of the devices is improved from 6.65% to 7.50% (13% enhancement). A detailed study of processing, characterization, microscopy, and device fabrication is conducted to understand the underlying mechanism for the enhanced device performance. The success of this work provides a simple and generally applicable approach to enhance light harnessing of low bandgap polymers in PSCs.