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428 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al. I SC ) enµE (43) ISC shows the maximum number of the photogenerated carriers that can be extracted from the solar cell. Hence, the external quantum efficiency, also known as incident photon to current efficiency (IPCE), can be calculated as: IPCE(λ) ) 1240 ISC λ Pin where Pin is an incident power at wavelength λ. Ifone electron-hole pair is created and separated per every absorbed photon, the ultimate ICPE is 100%. Generally speaking, the fate of photogenerated carriers is determined by the product of µτ (µ is the mobility of charges, and τ is the carrier lifetime), which has to be large enough to enable carrier separation before their recombination. ICPE can exceed 100% for devices generating more than one electron and hole per absorbed photon, for example, via impact ionization or carrier multiplication processes. Fill Factor (FF). The fill factor determines the quality of voltage-current characteristics. It is defined as the ratio of the maximum power Pmax under matched load conditions to the product of the open circuit voltage Voc and the shortcircuit current ISC: FF ) I m V m I SC V oc FF depends on the ability of charges to reach the electrodes when the driving force for carrier separation is lowered by the external bias. Typically, shunt resistances inside a solar cell account for a decrease in the fill factor. Power conversion efficiency (η) by definition is the maximum fraction of the input optical power converted into the electrical power: Table 5. Survey of Nanocrystal-Based Photovoltaic Devices a (44) (45) NC materials active layer design, electrodes η ) P max P in ) FF I OC V SC P in To compare different solar cells, special optical source simulating the sun spectrum is used as an input optical power. This source called AM1.5 (air mass of 1.5) was characterized by the power density of ∼1000 W/m 2 with the spectral intensity distribution matching that of sunlight at the earth’s surface at an incident angle of 48.2°. A full description of the AM1.5 standard spectrum can be found at http:// rredc.nrel.gov/solar/spectra/am1.5/. A typical experimental setup used to measure IPCE and other characteristics of a photovoltaic cell is shown in Figure 39. Table 5 provides a summary of NC-based photovoltaic devices reported in recent years. Different nanocrystalline materials and cell designs have been studied, but this work is currently far from completion. Below, we discuss several most promising directions in NC photovoltaics. 7.3.2. Nanocrystals in Hybrid Bulk Heterojunction Solar Cells Polymer solar cells based on conjugated organic polymers or polymer/fullerene blends are promising alternatives to conventional silicon-based technology. They comprise low cost fabrication by means of different printing techniques 470 with the flexible choice of various substrates. 471,472 These days, cells with single active layer can achieve power conversion efficiency of about 5%. 473,474 Recently, efficiency of ∼7% was demonstrated for a tandem solar cell with different band gaps. 475 The outdoor lifetime of such solar cell was found to be more than 1 year. 476 The main factors limiting the performance of polymer solar cells are chemical and photochemical stability of polymers and contacts as well as relatively poor electronic properties of organic materials. Typically used polymers show reasonable hole mobility of ∼0.1 cm 2 V -1 s -1 , while the electron mobilities are much lower (
Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 429 Figure 43. Hybrid nanorod-polymer solar cells based on the distributed network of heterojunction formed by poly(3-hexylthiophene) (P3HT) and CdSe NCs of various shapes (dots and rods). (a) Structure of regioregular P3HT. (b) The device structure including PEDOT: PSS coated ITO and Al electrodes. (c) Energy level diagram and charge separation in the P3HT/CdSe system. (d) IPCE spectra for photovoltaic cells containing CdSe dots (7 nm) and rods (7 × 30 and 7 × 60 nm). (e) I-V curve under AM1.5 illumination for a device containing 7 × 60 nm nanorods. Reprinted with permission from ref 457. Copyright 2002 American Association for Advancement of Science. the wide energy gap of the most semiconductor polymers. In this respect, semiconductor NCs are considered promising sensitizers, which allow one to extend the absorption into the red and near-IR range. The combination of NCs and conductive polymers was realized in so-called bulk heterojunction donor-acceptor solar cells, often called “hybrid solar cells”. The concept of bulk heterojunction implies blending of p- and n-type components to achieve efficient charge separation at interfaces, followed by charge transport through the percolation pathways. All electron-hole pairs are generated in close vicinity of a heterointerface and are promptly separated by transfer of one type of carrier (electron or hole) through the heterointerface. This approach is especially useful because of the short diffusion length of excitons in organic semiconductors. A typical example is a donor-acceptor combination of poly(3-hexylthiophene) (P3HT) 477 and soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. 455,472,478,479 Control over the nanoscale morphology of the blend is very critical for the device performance: interpenetrating networks and columnlike structures were found in efficient P3HT/PCBM devices by AFM, TEM, and small-angle X-ray studies. 473,480 Strong light absorption, excellent solution processability of colloidal NCs, and the possibility of fine-tuning the electronic structure motivate the integration of NCs into bulk heterojunction devices. In hybrid solar cells, NCs can be introduced as either p- or n-type components. 481 Various compositions containing Si, 466 CdSe, 457-460,482 CuInSe2, 483 ZnO, 484,485 CdS, 486 PbS, 462,487-490 PbSe, 491 and HgTe 461 NCs were tested in prototype solar cell structures. Below, we briefly discuss several characteristic examples. In 2002, Alivisatos and co-workers reported hybrid solar cell based on blends of P3HT and shape-engineered CdSe NCs (dots and rods), 457 forming donor-acceptor heterojunctions (Figure 43). To improve efficiency of carrier separation at the P3HT-CdSe interface, original hydrocarbon ligands were partially stripped off the CdSe NC surface and replaced by labile pyridine molecules. It was shown that CdSe nanorods allow one to achieve higher light conversion efficiencies as compared to spherical CdSe NCs due to the smaller number of interparticle hopping events necessary for electron to reach the collecting electrode. The maximum load of CdSe nanocomponent in its blend with P3HT could approach 90% by weight. The IPCE spectrum of the photovoltaic cell was determined by the absorption spectrum of CdSe component covering the entire visible spectrum from UV to ∼720 nm. Under simulated AM1.5 illumination, devices based on 7 × 60 nm CdSe rods exhibited promising power conversion efficiencies of ∼1.7%. Unfortunately, the tendency of nanorods to align parallel to substrate (i.e., ITO electrode) was unfavorable for efficient collection of the photogenerated electrons. CdSe tetrapods blended with poly(2-methoxy-5-(3′,7′-dimethyl-octyloxy)-p-phenylenevinylene), also known as OC1C10-PPV, demonstrated improved electron transport across the films. 460 The power conversion efficiency, however, was very similar (η ) 1.8% under AM1.5 illumination). The use of hyperbranched CdSe and CdTe NCs 396 allowed one to prevent the formation of phaseseparated domains and aggregates, a well-known problem for hybrid solar cells. Furthermore, large hyperbranched NCs can span throught the distance between anode and cathode electrodes, eliminating the need for rigorous control of the blend morphology (Figure 44). The resulting power conversion efficiencies of ∼2.2% were still lower than expected, presumably due to the limited penetration of polymer chains between densely packed branches of CdSe component. The light conversion efficiencies comparable to the CdSe/P3HT cells were obtained using ZnO NCs as electron acceptor and P3HT (η ≈ 0.9%) 484,485 or poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV, η ≈ 1.4%). 485 Narrow gap semiconductor NCs have been used in hybrid photovoltaic cells to harvest infrared part of solar spectrum. So far, reported hybrid solar cell devices using HgTe, 461 PbS, 462,488,490 and PbSe 492 NCs showed power conversion efficiencies below 1%, presumably due to inappropriate surface chemistry (traps, insulating nature of surface ligands) and partial phase separation of the blend components. The crucial role of surfaces in NC solar cells had to
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