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

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%.

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Polymer solar cells with enhanced open-circuit voltage and efficiency

An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high-performance low-bandgap polymers was the major limiting factor for achieving high-performance tandem solar cell. Here we report the development of a high-performance low bandgap polymer (bandgap 60% and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency under standard reporting conditions (25 °C, 1,000 Wm(-2), IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.

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A polymer tandem solar cell with 10.6% power conversion efficiency

Perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies, exhibiting rapidly rising efficiencies. This is likely to continue to rise, but in the development of these solar cells there are unusual characteristics that have arisen, specifically an anomalous hysteresis in the current–voltage curves. We identify this phenomenon and show some examples of factors that make the hysteresis more or less extreme. We also demonstrate stabilized power output under working conditions and suggest that this is a useful parameter to present, alongside the current-voltage scan derived power conversion efficiency. We hypothesize three possible origins of the effect and discuss its implications on device efficiency and future research directions. Understanding and resolving the hysteresis is essential for further progress and is likely to lead to a further step improvement in performance.

Anomalous hysteresis in perovskite solar cells

This article is written from an organic chemist's point of view and provides an up-to-date review about organic solar cells based on small molecules or oligomers as absorbers and in detail deals with devices that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an acceptor (n-type semiconductor) material. The article pays particular attention to the design and development of molecular materials and their performance in corresponding devices. In recent years, a substantial amount of both, academic and industrial research, has been directed towards organic solar cells, in an effort to develop new materials and to improve their tunability, processability, power conversion efficiency, and stability. On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches to enhancing the efficiency of organic solar cells are analyzed.

Small molecule organic semiconductors on the move: promises for future solar energy technology.

A tandem polymer photovoltaic device includes a first bulk hetero-junction polymer semiconductor layer, a second bulk hetero-junction polymer semiconductor layer spaced apart from the first bulk hetero-junction polymer semiconductor layer, and a metal-semiconductor layer between and in contact with the first and second bulk hetero junction polymer semiconductor layers. The first and second bulk hetero-junction polymer semiconductor layers have complementary photon absorption spectra.

Highly efficient tandem polymer photovoltaic cells

A kink is sometimes seen in the I-V curves for organic solar cells. In literature charge blocking has been speculated to be responsible for such kind of anomalous features. In this manuscript, we use poly(3-hexylthiophene):[6, 6]-phenyl-C61-butyric acid methyl ester as our model polymer system and investigate different device structures using ultraviolet photoelectron spectroscopy as our primary tool to investigate the reason for this S-shaped kink. We attribute this anomalous feature to the presence of strong interface dipoles. We further propose a model based on the standard set of Poisson equation, continuity equation, and current density equations including both drift and diffusion components.

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Dipole induced anomalous S-shape I-V curves in polymer 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-

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

In this article, we review general approaches, challenges and solutions, as well as latest research progress, in the field of polymer tandem solar cells. Despite many limiting factors ranging from processing issues to characterization issues, it has been shown that high efficiency and in-depth understanding of tandem cell operation can be achieved via smart device design and analysis, and interface engineering. We further discuss in detail the operation principles and design rules of polymer tandem cells, revealing a bright future to reach double-digit power conversion efficiency from polymer tandem cells.

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Tandem polymer photovoltaic cells—current status, challenges and future outlook

With the aim of revealing the role of additives in polymer solar cells, different amounts of 1,8-octanedithiol (OT) were added to the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend system in order to observe the transition between systems with and without additive. We found that highly efficient P3HT:PCBM networks can be formed in the very early stage of the spin-coating process when a small amount of additive was added. The carrier mobilities of this fast-grown network were found to be comparable with those processed with solvent annealing. As a result, short circuit current density (Jsc) as high as ∼9 mA cm−2 can be obtained and the fill factor (FF) can reach ∼62%. The power conversion efficiency (PCE), however, is limited by the low open circuit voltage (Voc) of the devices processed with OT. According to the grazing incidence X-ray diffraction data, a much shorter interlayer spacing (d(100) ∼ 15.6−15.7 A) compared with those processed with different methods i...

https://yylab.seas.ucla.edu/papers/jp810798z.lowlink.pdf_v03.pdf

Srinivas Sista

Papers

Highly efficient tandem polymer photovoltaic cells

Dipole induced anomalous S-shape I-V curves in polymer solar cells

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

Tandem polymer photovoltaic cells—current status, challenges and future outlook

Fast-Grown Interpenetrating Network in Poly(3-hexylthiophene): Methanofullerenes Solar Cells Processed with Additive