Recent development of the inverted configuration organic solar cells

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Abstract

Recent years, the power conversion efficiency (PCE) of normal configuration organic solar cells (OSCs) has obtained rapid progress to reach more than 6% under standard illumination, which is reasonable value for the commercial criterion. More and more research attention has been paid on the stability and lifetime of OSCs. A novel structural OSCs with high work function metal or metal oxide as the top electrode and low work function metal as the bottom anode is proposed and named as inverted configuration OSCs. The inverted configuration OSCs with high work function metal as top cathode could improve OSCs's lifetime, i.e., protecting cells from the damage by oxygen and moisture in air. Furthermore, the inverted configuration OSCs is the appealing alternative to the conventional regular structure due to the inherent vertical phase separation in the polymer active layers and high stability or long device lifetime. The inverted configuration OSCs have not only achieved an impressive PCE of 4.4%, but also exhibited an exceptional device lifetime without encapsulation. In this review article, the recent developments and vital researches on the inverted configuration OSCs are summarized.

Graphical abstract

A novel structural organic solar cells (OSCs) with high work function metal as the top electrode and low work function metal or metal oxide as the bottom anode was proposed and named as inverted configuration OSCs. In this review article, the recent developments and vital researches on the inverted configuration OSCs are summarized.

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Introduction

Recent years, more and more research attention has been paid to the development of organic solar cells (OSCs) due to its rapid efficiency improvement, the cost effective energy harvesting, ease of processing, and compatibility with flexible substrates. The power conversion efficiency (PCE) of polymer solar cells has reached as high as ∼6% [1], [2]. Many effective approaches have been carried out to improve the performance of solar cells through the synthesis of new narrow bandgap materials for better photon harvesting, optimization of phase segregation in the bulk heterojunction (BHJ) layers, interfacial modification for better charge carrier collection, and design of novel configuration cells [3], [4], [5], [6], [7], [8]. However, many basic physical and mechanical problems, such as the interfacial states between organic electron donor/acceptor, between organic materials/inorganic semiconductor, and between organic materials/metal, stability/degradation of cells, morphology control, and the role of ultrathin interlayer, are still not very clear [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. The current research pursuits for OSCs concentrate in obtaining high efficiency, stability, low-cost and high-speed production in individual cells. The ultimate realization of all these goals in one single cell is a great challenge and no such report has been published. In order to improve the PCE of OSCs, the key parameters, such as short circuit current (Jsc), open circuit voltage (Voc), Fill factor (FF), series and shunt resistance, should be optimized by improving the photon harvesting, interfacial engineering and different treatments on substrates or active layers.

The normal configuration OSCs are made of an active BHJ layer or planner heterojunction layers sandwiched by a high work function and transparent metal oxide as the bottom anode and a low work function metal as the top cathode. For efficient charge collection, work functions of anode and cathode should be matched to the highest occupied molecular orbits (HOMO) of donor and the lowest unoccupied molecular orbits (LUMO) of acceptor, respectively. Low work function metals as the top cathode have been selected for better matching with the LUMO of acceptor. However, metals with low work function are not very stable as the top electrode due to their sensitivity to oxygen and moisture in air [21], [22]. In order to protect top electrode, bi-layered metals such as Ca/Al or Mg/Al have been used as composite cathode for the performance improvement in OSCs and organic light-emitting diodes (OLEDs) [23], [24]. Inverted configuration OSCs with low work function metals modified ITO as the transparent cathode and a high work function metal as anode could effectively avoid the device degradation induced by the contamination of metal cathodes. The inverted configuration OSCs also avoids the need for using poly (styrene sulfonic acid) doped poly (3,4-ethylenedioxythiophene) PEDOT:PSS at ITO interface, which has been shown to degrade performance due to the effect of strong acidic nature of PSS on the ITO surface [25]. The common used high work function metal electrodes like Ag and Au are effective in hole collection; however, the work function of Au may decrease from 5.4 to 4.7 eV in the air due to the contamination by carbon and oxide, which will lead to the degradation of the device's performance [26]. Nevertheless, in the case of the Ag electrode, the oxidization of Ag will increase its work function to from 4.3 to 5.0 eV, which will benefit for hole collection from electron donor materials to Ag and improve the cell's performance [27]. It is also expected that the inverted configuration has the advantage over the normal configuration because of the P3HT:PCBM vertical phase separation [28]. 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 normal configuration OSCs performance and makes the inverted configuration OSCs as a promising choice. Recent years, the efficiency gap between normal and inverted structural solar cells has been rapidly dwindled. Yang' group reported that the efficiency of inverted configuration OSCs has more than 4% under standard measurement conditions [29]. Recently, Hsieh et al. [30] reported that an inverted configuration OSCs has not only achieved an impressive PCE of 4.4%, but also exhibited an exceptional device lifetime without encapsulation.

The ideal fabrication process for OSCs will be solution processing, in which different cell layers are deposited onto flexible substrates to tailor for the ultimate simple roll-to-roll type printing. Additionally, the OSCs should maintain high device stability and efficiency over a certain period of time. The inverted configuration OSCs may be the best candidate to meet all these requirements including high efficiency, stability, low-cost and high-speed production into one system. Recently, the inverted device architecture has been investigated as a suitable architecture tailoring for solution processing, which allows various cell layers to deposit onto flexible substrates and is promising for scale-up production via industrial roll-to-roll type fabrication.

This review article is organized as follows: following a brief introduction on the mechanism of OSCs in Section 2. An overview of commonly used materials is provided in Section 3. Section 4 is devoted to introduce the latest achievement on the inverted configuration OSCs and discuss the probable routes for efficiency improvement of inverted configuration OSCs. Finally, we conclude and offer our perspective on the challenges to achieve high efficiency and long device lifetime in Section 6.

Section snippets

Operational mechanism

Organic solar cells have attracted a significant amount of attention due to the need to develop an inexpensive clean and sustainable renewable energy source. No matter what the cell structures are, the operational mechanism is the same. Inverted configuration is mainly directed to solve the issue of device lifetime and stability. The operation of OSCs mainly involves the following five steps: (i) the light absorption by the active layer; (ii) the formation of exciton and subsequent diffusion to

Small molecular materials

Generally, organic semiconductors have low dielectric constant (ε≈3) and high absorption coefficient of (>105 cm−1), which means that thin layer can still be highly absorptive while simultaneously preserving good charge transport performance. Organic semiconductor could be classified as small molecule and polymer according to its molecular weight, or electron donor and electron acceptor according to its electron gaining and loss ability. When a photon was absorbed by an electron donor, the

Inverted configuration small molecular OSCs

The commonly used organic small molecular materials are metal phthalocyanine and C60 as electron donor and electron acceptor, respectively. In a conventional small molecule OSCs, a buffer layer, such as bathocuproine (BCP), bathophenanthroline (Bphen), 1,3,5-tris(2-N-phenylbenzimidizolyl) benzene (TPBi), and tris-8-hydroxy-quinolinato aluminum (Alq3), is inserted between the acceptor and the cathode to prevent exciton quenching [60], [61], [62], [63]. So far inverted small molecule solar cells

Recent progress in potential application

Although vacuum deposition is used industrially in packaging industry and OLEDs fabrication, the vacuum steps lead to a low processing speed, thus limiting the space in production cost cutting. Recently, Zou et al. reported the fabrication of inverted structured PSCs using metal grid/conducting polymer hybrid transparent electrode to replace ITO [127]. The performance of the devices could be easily tuned by varying the width and interval of the metal grids. For the narrowly intervened Ag grids,

Conclusion

The current performance of OSCs technology is limited (as compared to silicon-based solar cells) by mainly two factors: (i) the efficiency, which is on the order of 5% (although a few reports of efficiencies approaching higher values in the 6–8% range have appeared recently) [136], [137], [138], and (ii) the lifetime, which is on the order of years under outdoor conditions with good stability being recorded for both air stable devices and encapsulated devices [123], [124]. The inverted

Acknowledgement

The authors express their thanks to the supports from the National Natural Science Foundation of China (Grant nos. 10804006, 20904057 and 21074055), the National Natural Science Funds for Distinguished Young Scholar (Grant no. 60825407), Natural Science Foundation of Beijing (Grant no. 1102028), the 111 Project (Grant no.B08002), Basic Research Foundation of Beijing Jiaotong University (2011JBM123) and NUST Research Funding (no. 2010ZDJH04), Beijing Municipal Science & Technology Commission (

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