Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based first-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.

Efficient planar heterojunction perovskite solar cells by vapour deposition

The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

Conjugated polymer-based organic solar cells

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organicdevices have not yet reached those of their inorganic counterparts (η ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400094498

Organic solar cells: An overview

Chalcopyrite copper indium sulfide (CuInS2) and copper indium gallium selenide (Cu(InxGa1-x)Se2; CIGS) nanocrystals ranging from ∼5 to ∼25 nm in diameter were synthesized by arrested precipitation in solution. The In/Ga ratio in the CIGS nanocrystals could be controlled by varying the In/Ga reactant ratio in the reaction, and the optical properties of the CuInS2 and CIGS nanocrystals correspond to those of the respective bulk materials. Using methods developed to produce uniform, crack-free micrometer-thick films, CuInSe2 nanocrystals were tested in prototype photovoltaic devices. As a proof-of-concept, the nanocrystal-based devices exhibited a reproducible photovoltaic response.

Synthesis of CuInS2, CuInSe2, and Cu(InxGa1-x)Se2 (CIGS) Nanocrystal “Inks” for Printable Photovoltaics

We report on solution-processed hybrid solar cells consisting of a nanocrystalline inorganic semiconductor, CuInS2, and organic materials. Synthesis of quantized CuInS2 nanoparticles was performed using a colloidal route, where the particle surface was shielded by an organic surfactant. First attempts were made to use nanocrystalline CuInS2 with fullerene derivatives to form flat-interface donor–acceptor heterojunction solar cells. We investigated also bulk heterojunctions by replacing the CuInS2 single layer by a blend of CuInS2 and p-type polymer (PEDOT:PSS; poly(3,4-ethylenedioxythiophene:poly(styrene sulfonic acid) in the same cell configuration. Bulk heterojunction solar cells show better photovoltaic response with external quantum efficiencies up to 20 %.

Hybrid Solar Cells Based on Nanoparticles of CuInS2 in Organic Matrices

Hybrid solar cells based on inorganic nanoclusters and conjugated polymers

Hybrid solar cells based on CdSe nanoparticles and a PPV-type polymer containing fluorene and thiophene units (PFT) were investigated. The CdSe/PFT devices showed very low photocurrent and fill factor values, which was attributed to the poor charge transport in the TOPO-capped CdSe nanoparticle phase. Thus, ternary systems based on mixtures of PFT/CdSe and the fullerene derivative PCBM were investigated. The CdSe:PCBM ratio was varied, and nanoparticles with different sizes were also used. It was observed that for the optimized composition of 20 wt% PFT + 40 wt% CdSe + 40 wt% PCBM the devices presented higher photocurrents and efficiencies. The photophysical and electrochemical properties and microscopy images (AFM and HRTEM) of the ternary systems were systematically investigated to elucidate the mechanism of action of the inorganic nanoparticles in these ternary hybrid devices.

The effects of CdSe incorporation into bulk heterojunction solar cells

Hybrid materials consist of inorganic nanoparticles embedded in polymer matrices. An advantage
of these materials is to combine the unique properties of one or more kinds of inorganic nanoparticles
with the film forming properties of polymers. Most of the polymers can be processed from solution at room
temperature enabling the manufacturing of large area, flexible and light weight devices. To exploit the full
potential for the technological applications of the nanocrystalline materials, it is very important to endow
them with good processing attributes. The surface of the inorganic cluster can be modified during the synthesis
by organic surfactants. The surfactant can alter the dispersion characteristic of the particles by initiating
attractive forces with the polymer chains, in which the particles should be homogenously arranged. In this
review, we present wet chemical methods for the synthesis of nanoparticles, which have been used as photovoltaic
materials in polymer blends. The photovoltaic performance of various inorganic/organic hybrid solar
cells, prepared via spin-coating will be the focus of this contribution.

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Core/shell nanomaterials in photovoltaics

We investigated blends of poly hexylthiophene (P3HT) with copper indium diselenide nanocrystals for photovoltaic applications. Depending on the synthesis, the particles were shielded by different organic surfactants. Different concentrations of these nanoparticles were suspended in the polymer solutions and spin cast onto ITO glass. Morphological studies have been performed by atomic force microscopy and transmission electron microscopy. Films consisting of tri-n-octylphosphine oxide-capped CISe:P3HT show photovoltaic response. The best performance we obtained is an open-circuit voltage of about 1 V and a photocurrent of 0.3 mA/cm2 using a white light illumination intensity of 80 mW/cm2.

Elif Arici

Papers

Hybrid Solar Cells Based on Nanoparticles of CuInS2 in Organic Matrices

Hybrid solar cells based on inorganic nanoclusters and conjugated polymers

The effects of CdSe incorporation into bulk heterojunction solar cells

Core/shell nanomaterials in photovoltaics

Morphology effects in nanocrystalline CuInSe 2 -conjugated polymer hybrid systems