Bulk heterojunction (BHJ) polymer solar cells (PSCs) based on composites of conjugated polymers (electron donor) and fullerene derivatives (electron acceptor) have attracted attention due to their potential as renewable energy sources. [1 -7 ] The major challenges for BHJ solar cells are the achievement of competitive power conversion effi ciencies (PCEs) and the demonstration of long-term air stability. [8][9][10][11][12][13][14][15][16] BHJ solar cells are typically fabricated with a transparent conductive anode (e.g. indium tin oxide, ITO), a low-work-function metal cathode (e.g., Al, Ca), and an active layer (a mixture of conjugated polymer and fullerene derivative) sandwiched between the anode and cathode. The BHJ layer and cathode dramatically affect the stability. In particular, the cathode is susceptible to degradation by oxygen and water vapor. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is often used as an anode buffer layer. Long-term stability is a problem because PEDOT:PSS is hygroscopic and acidic. [17][18][19][20][21] In order to circumvent these problems, inverted polymer solar cells have been developed; air-stable high-work-function metals (e.g., Au, Ag) are used as the anode to collect holes and ITO is used as the cathode to collect electrons. In the inverted architecture, n-type metal oxides such as titanium oxide (TiO x ), zinc oxide (ZnO), and cesium carbonate (Cs 2 CO 3 ) are deposited onto the ITO electrode to break the symmetry. [ 22 − 24 ] The elimination of the PEDOT:PSS layer improves the device stability. Moreover, in the inverted cell, the anode is a highwork-function metal such as Ag, which can be formed using coating or printing technology to simplify and lower the cost of manufacturing. [ 25 ] Among the n-type metal oxides used in inverted cells, ZnO is a promising candidate due to its relatively high electron mobility, environmental stability, and high transparency. A variety of fabrication methods have been employed to grow thin fi lms of ZnO. Sol-gel method has been extensively investigated as a solution-based thin-fi lm deposition process. [ 26 ] Sol-gelderived ZnO fi lm is widely used in inverted solar cells. However, a high annealing temperature, usually over 200 ° C and incompatible with fl exible substrates, is used to promote crystallization and removal of residual organic compounds. [27][28][29] Although solution-processed ZnO nanoparticles have been shown to be easily processed into thin fi lms via spin coating or roll-to-roll printing at room temperature, [ 23 , 30 , 31 ] ZnO nanoparticles are not very stable in solution and a ligand is usually used to stabilize them. [ 32 ] We report here that uniform sol-gel-derived ZnO fi lms can be obtained at relatively low annealing temperatures ( ≤ 200 ° C) and they can function as the effi cient electron transporting layer in inverted solar cells.Despite a dramatic improvement of stability, inverted solar cells suffer from relatively lower PCEs compared to conventional solar cells, mainly due to the ...

Research relating to organic solar cells based on solution‐processed, bulk heterojunction (BHJ) films has been dominated by polymeric donor materials, as they typically have better film‐forming characteristics and film morphology than their small‐molecule counterparts. Despite these morphological advantages, semiconducting polymers suffer from synthetic reproducibility and difficult purification procedures, which hinder their commercial viability. Here, a non‐polymeric, diketopyrrolopyrrole‐based donor material that can be solution processed with a fullerene acceptor to produce good quality films is reported. Thermal annealing leads to suitable phase separation and material distribution so that highly effective BHJ morphologies are obtained. The frontier orbitals of the material are well aligned with those of the fullerene acceptor, allowing efficient electron transfer and suitable open‐circuit voltages, leading to power conversion efficiencies of 4.4 ± 0.4% under AM1.5G illumination (100 mW cm−2). Small molecules can therefore be solution processed to form high‐quality BHJ films, which may be used for low‐cost, flexible organic solar cells.

The power conversion efficiencies of bulk heterojunction (BHJ) solar cells can be increased from 5 to 6.5% by incorporating an ultrathin conjugated polyelectrolyte (CPE) layer between the active layer and the metal cathode. Poly[N-9''-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C(71) butyric acid methyl ester (PC(71)BM) were chosen for the photoactive layer. CPEs with cationic polythiophenes, in both homopolymer and block copolymer configurations, were used to improve the electronic characteristics. The significant improvement in device performance and the simplicity of fabrication by solution processing suggest a promising and practical pathway for improving polymer solar cells with high efficiencies.

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Bulk heterojunction solar cells are fabricated from blends of oligothiophene with a dialkylated diketopyrrolopyrrole chromophore:[6,6]-phenyl C71 butyric acid methyl ester. Absorption and photocurrent of the films extend to 800 nm. A power conversion efficiency (PCE) of 3.0% is obtained under simulated 100 mW/cm2 AM1.5 illumination with a 9.2 mA/cm2 short-circuit current density and an open-circuit voltage of 0.75 V. The hole and electron mobilities in the 50:50 blend are fairly balanced, 1.0×10−4 and 4.8×10−4 cm2/V s, respectively. This is the highest PCE reported to date for solar cells using solution processable small molecules.

Golden solar cells: Several positive effects arise from the addition of truncated octahedral Au nanoparticles (ca. 70 nm diameter) to bulk heterojunction (BHJ) photovoltaic cells fabricated from a variety of donor polymers and PC70BM as acceptor (see picture). At the optimized blend ratio of Au nanoparticles (5 wt %) in the active layer, the power conversion efficiency increased for all polymer/PC70BM systems under study.

Photoinduced electron transfer is observed in polymer bulk heterojunction solar cells with very small interfacial energy offset. The results imply that open circuit voltage values close to the band gap of the semiconducting polymer should be possible for polymer bulk heterojunction solar cells just as for inorganic solar cells.

Donor-acceptor polymer-fullerene-based bulk heterojunction (BHJ) photovoltaic cells have many advantages; specifi cally, they can be fabricated in large-areas with simple solution methods involving a low-cost roll-to-roll manufacturing process. [1][2][3][4][5][6][7] The active layer in BHJ solar cells provides charge generation, separation, transport, and collection. During these steps, sweep-out of photo-generated carriers competes with charge recombination within the BHJ due to low carrier mobility. [8][9][10] Blocking the recombination is a key method of increasing device efficiency. [ 11 ] Therefore, we need to decrease the thickness of the active layer of BHJ fi lm and maximize its absorption capability to reduce the likelihood of recombination and thereby improve the effi ciency of the device.Metallic silver (Ag) nanoparticles (NPs) can be effective in thin fi lm solar cells because of their localized surface plasmon resonance, which can increase the light absorption capability of an active layer within a range of wavelengths. Thus, the addition of metal NPs into a BHJ active layer can potentially enhance the absorption and increase the photo-generation of mobile carriers. Several groups have recently reported that, when combined, polymer and metal NPs improve light absorption and cell effi ciency. [12][13][14][15] To ensure that metal NPs have positive effects in organic photovoltaic devices (OPVs), we must diminish the possibility of transferring nonradiative energy, which quenches excitons in the BHJ active layer. [ 12 , 15 ] Because of this concern, most researchers add metal NPs to the interface of the indium tin oxide (ITO) anode and the BHJ active layer; alternatively, they embed the metal NPs in a poly(3,4-ethylene dioxythiophene:poly(styrene sulfonate) (PEDOT:PSS) hole transporting interlayer to induce plasmon excitation within the thin BHJ fi lm. [ 12 , 16 , 17 ] However, Kim and Carroll directly added Ag or Au NPs to a BHJ polymer and confi rmed that the cell effi ciency was enhanced by the improved electrical conductivity. [ 13 ] The simple method of directly mixing metal NPs in an active layer is attractive because it enhances the overall performance of the cell. Specifi cally, this method can reduce the device resistance; [ 13 ] furthermore, the incident light can be refl ected and scattered by the embedded metal NPs while passing through the active layer, thereby increasing the optical path length of the incident light.Our study demonstrates several positive effects that arise from the addition of Ag NPs with controlled diameter size in BHJ photovoltaic cells fabricated with poly[N-9"-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT)/ [6,6]-phenyl C 70 butyric acid methyl-ester (PC 70 BM). Using effective, easily processable solution chemistry of a polyol process, we can synthesize Ag particles with wellcontrolled sizes and well-defi ned shapes. Specifi cally, we produced particles with diameters of 30 nm, 40 nm, and 60 nm. These Ag NPs have differe...