This Review summarizes recent progress in the development of polymer solar cells. It covers the scientific origins and basic properties of polymer solar cell technology, material requirements and device operation mechanisms, while also providing a synopsis of major achievements in the field over the past few years. Potential future developments and the applications of this technology are also briefly discussed.

Polymer solar cells

typically based on n-type metal oxides, our device is solutionprocessed at room temperature, enabling easy processibility over a large area. Accordingly, the approach is fully amenable to highthroughput roll-to-roll manufacturing techniques, may be used to fabricate vacuum-deposition-free PSCs of large area, and find practical applications in future mass production. Moreover, our discovery overturns a well-accepted belief (the inferior performance of inverted PSCs) and clearly shows that the characteristics of high performance, improved stability and ease of use can be integrated into a single device, as long as the devices are optimized, both optically and electrically, by means of a meticulously designed device structure. We also anticipate that our findings will catalyse the development of new device structures and may move the efficiency of devices towards the goal of 10% for various material systems. Previously, we reported that PFN can be incorporated into polymer light-emitting devices (PLEDs) to enhance electron injection from high-work-function metals such as aluminium (work function w of 4.3 eV) 22,23 and has thus been used to realize high-efficiency, air-stable PLEDs 24 . Furthermore, we also found that efficient electron injection can be obtained even in the most noble metals with extremely high work functions, such as gold (w ¼ 5.2 eV), by lowering the effective work function (for example lowering w in gold by 1.0 eV), which has previously been ascribed to the formation of a strong interface dipole 25 .

/pdf/enhanced-power-conversion-efficiency-in-polymer-solar-cells-4oh1re6qpi.pdf

Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure

Sun is the largest carbon-neutral energy source that has not been fully utilized. Although there are solar cell devices based on inorganic semiconductor to efficiently harvest solar energy, the cost of these conventional devices is too high to be economically viable. This is the major motivation for the development of organic photovoltaic (OPV) materials and devices, which are envisioned to exhibit advantages such as low cost, flexibility, and abundant availability. [1] The past success in organic light-emitting diodes provides scientists with confidence that organic photovoltaic devices will be a vital alternate to the inorganic counterpart. At the heart of the OPV technology advantage is the easiness of the fabrication, which holds the promise of very low-cost manufacturing process. A simple, yet successful technique is the solution-processed bulk heterojunction (BHJ) solar cell composed of electron-donating semiconducting polymers and electron-withdrawing fullerides as active layers. [2] The composite active layer can be prepared as a large area in a single step by using techniques such as spin-coating, inkjet-printing, spraycoating, gravure-coating, roller-casting etc. [3] In the last fifteen years, a significant progress has been made on the improvement of the power-conversion efficiency (PCE) of polymer BHJ solar cells, and the achieved efficiencies have evolved from less than 1% in the poly(phenylene vinylene) (PPV) system in 1995, [2] to 4‐5% in the poly(3-hexylthiphene) (P3HT) system in 2005, [4] to around 6%, as reported recently. [5] However, the efficiency of polymer solar cells is still significantly lower than their inorganic counterparts, such as silicon, CdTe and CIGS, which prevents practical applications in large scale.

For the Bright Future—Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%

Solution-processed bulk-heterojunction solar cells have gained serious attention during the last few years and are becoming established as one of the future photovoltaic technologies for low-cost power production. This article reviews the highlights of the last few years, and summarizes today's state-of-the-art performance. An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic technology towards the magic 10% regime. A cost model supplements the technical discussions, with practical aspects any photovoltaic technology needs to fulfil, and answers to the question as to whether low module costs can compensate lower lifetimes and performances.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.200801283

Polymer‐Fullerene Bulk‐Heterojunction Solar Cells

Following the development of the bulk heterojunction1 structure, recent years have seen a dramatic improvement in the efficiency of polymer solar cells. Maximizing the open-circuit voltage in a low-bandgap polymer is one of the critical factors towards enabling high-efficiency solar cells. Study of the relation between open-circuit voltage and the energy levels of the donor/acceptor2 in bulk heterojunction polymer solar cells has stimulated interest in modifying the open-circuit voltage by tuning the energy levels of polymers3. Here, we show that the open-circuit voltage of polymer solar cells constructed based on the structure of a low-bandgap polymer, PBDTTT4, can be tuned, step by step, using different functional groups, to achieve values as high as 0.76 V. This increased open-circuit voltage combined with a high short-circuit current density results in a polymer solar cell with a power conversion efficiency as high as 6.77%, as certified by the National Renewable Energy Laboratory. Adding electron-withdrawing groups to the backbone of the polymer PBDTTT is shown to increase the open-circuit voltage of photovoltaic cells, resulting in a polymer solar-cell that has a certified power-conversion efficiency of 6.77%.

/pdf/polymer-solar-cells-with-enhanced-open-circuit-voltage-and-464500qqzd.pdf

Polymer solar cells with enhanced open-circuit voltage and efficiency

The self-organization of the polymer in solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) is studied systematically as a function of the spin-coating time ts (varied from 20–80 s), which controls the solvent annealing time ta, the time taken by the solvent to dry after the spin-coating process. These blend films are characterized by photoluminescence spectroscopy, UV-vis absorption spectroscopy, atomic force microscopy, and grazing incidence X-ray diffraction (GIXRD) measurements. The results indicate that the π-conjugated structure of RR-P3HT in the films is optimally developed when ta is greater than 1 min (ts ∼ 50 s). For ts < 50 s, both the short-circuit current (JSC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < ts < 55 s, the JSC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered π-conjugated structure of RR-P3HT to 1.2 % for a less-ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon-to-electron conversion efficiency spectrum of the less-ordered solar cell shows a clear loss in both the overall magnitude and the long-wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under “solvent annealing” conditions, the polymer is seen to be ordered even at 67 wt % PCBM loading. The open-circuit voltage (VOC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.

“Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3‐hexylthiophene) and Methanofullerenes

A method which enables the investigation of the buried interfaces without altering the properties of the polymer films is used to study vertical phase separation of spin-coated poly(3-hexylthiophene) (P3HT):fullerene derivative blends. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis reveals the P3HT enrichment at the free (air) surfaces and abundance of fullerene derivatives at the organic/substrate interfaces. The vertical phase separation is attributed to the surface energy difference of the components and their interactions with the substrates. This inhomogeneous distribution of the donor and acceptor components significantly affects photovoltaic device performance and makes the inverted device structure a promising choice.

/pdf/vertical-phase-separation-in-poly-3-hexylthiophene-fullerene-j8mjsc045s.pdf

Vertical Phase Separation in Poly(3‐hexylthiophene): Fullerene Derivative Blends and its Advantage for Inverted Structure Solar Cells

Vertical phase separation in poly(3-hexylthiophene): Fullerene derivative blends and its advantage for inverted structure solar cells

Adv. Mater. 2009, 21, 1–5 2009 WILEY-VCH Verlag Gmb Polymer solar cells have evolved as a promising costeffective alternative to inorganic-based solar cells due to their potential to be low-cost, light-weight, and flexible. Since the discovery of ultrafast photoinduced charge transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, intensive research with potential materials has been carried out as future photovoltaic (PV) technology. Two organic materials with distinct donor and acceptor properties are required to form a heterojunction in the bulk film, which is often achieved by solution processing. In such a case, the BHJ not only provides abundant donor/acceptor interfaces for charge separation, but also forms an interpenetrating network for charge transport. Highly efficient polymer solar cells based on poly(3hexylthiosphene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) have been reported with power conversion efficiencies of 4–5%. The two most decisive parameters regarding polymer-solar-cell efficiencies are the open-circuit voltage (Voc) and the short-circuit current (Jsc). Jsc is mostly determined by the light absorption ability of the material, the charge-separation efficiency, and the high and balanced carrier mobilities. On the other hand, Voc is limited by the difference in the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, where a small Voc (as compared to the photon energy) represents a smaller driving force for the PV process. For the P3HT:PC61BM system, the Voc is around 0.6 V, which significantly limits the overall device efficiency. An effective method to improve the Voc of polymer solar cells is to manipulate the HOMO level of the donor and/or LUMO level of the acceptor. Until now, fullerene derivatives have proved to be one of the best and most commonly used electron acceptors. Fortunately, it is convenient to change the band gap and energy levels of the donormaterial bymodifying the chemical structure to achieve a high Voc. [13,14] Amongst various polymers, poly{[2,7-(9-(20-ethylhexyl)-9-hexylfluorene])-alt-[5,50-(40, 70-di-2-thienyl-20,10,30-benzothiadiazole)]} (PFDTBT) has a deep HOMO level, which leads to a large Voc when blended with PC61BM. Svensson et al. [15] have reported polymer PV cells with a Voc of 1V based on alternating copolymer PFDTBT blendedwith PC61BM.Moreover, Inganas et al. [16] reported a systematic study of PV cells using four different fluorene copolymers by varying the length of the alkyl side chain and chemical structure, exhibiting power conversion efficiencies above 2–3%. Unfortunately, in their case, the low photocurrent becomes a major limiting factor in achieving higher efficiencies, suggesting low carrier mobilities. In this study, poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt[5,5-(4,7-di-20-thienyl-2,1,3-benzothiadiazole)]} (BisEH-PFDTBT) and poly{[2,7-(9,9-bis-(3,7-dimethyl-octyl)-fluorene)]-alt-[5,5-(4,7di-20-thienyl-2,1,3-benzothiadiazole)]} (BisDMO-PFDTBT), which have the same polymer backbone as PFDTBT but different side chains, were studied in order to achieve higher efficiency values, as well as to investigate the side-chain effects. BisDMO-PFDTBT has proven to be a promising candidate as a donor material for high efficiency polymer BHJ solar cells. Under simulated solar illumination of AM 1.5G (100mWcm ), the BisDMO-PFDTBT blended with (6,6)-phenyl-C71-butyric acidmethyl ester (PC71BM) achieveda maximum power conversion efficiency (PCE) of up to 4.5% with a thin active-layer thickness of only 47 nm. The device exhibited an open-circuit voltage (Voc) of 1V, a short-circuit current (Jsc) of 9.1mAcm , and a reasonably high external quantum efficiency (EQE) exceeding 50% over the entire visible range, with an EQE maxima of 67% at 380nm. When considering the polymer design on the molecular level, the two main factors relating to the ease of material processibility and PV performance are the polymer solubility in common organic solvents and the hole mobility, respectively. In order to obtain good solubility, it is important to have long side chains attached on the polymer backbone. Note that in our experiment, the attached side chains are saturated alkyl groups that have little influence on the molecular energy levels of the donor material. Therefore, it allows us to explore the interchain interaction, particularly the charge-hopping effect. In addition, bulky side chains have a negative effect on the carrier mobility, since interchain hopping of charge carriers requires a favorable overlapping of the electron wave function of adjacent conjugated units on the polymer main chains. Apparently, if the non-

Guanwen Yang

Papers

Polymer solar cells with enhanced open-circuit voltage and efficiency

“Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3‐hexylthiophene) and Methanofullerenes

Vertical Phase Separation in Poly(3‐hexylthiophene): Fullerene Derivative Blends and its Advantage for Inverted Structure Solar Cells

Vertical phase separation in poly(3-hexylthiophene): Fullerene derivative blends and its advantage for inverted structure solar cells

Efficient Polymer Solar Cells with Thin Active Layers Based on Alternating Polyfluorene Copolymer/Fullerene Bulk Heterojunctions