**5. Summary and outlook**

economic potential of OPV technology to estimate the cost of the OPV electricity [4, 27-29, 149-150]. One key factor that dominates the cost is definitely the power conversion efficiency. Figure 10 shows the PCE values of the reported OPV devices vs. the area (top) and the year (bottom) by Krebs et al.[30] It is found that most of the highly promising efficiency data are

**Figure 10.** Top: A plot of the power conversion efficiency versus the active area. Bottom: The PCE values obtained ver‐

sus the publication year. [30] Copyright Elsevier 2013.

374 Solar Cells - New Approaches and Reviews

The concept of bulk hetero junction solar cells has been continuously developed over the past two decades. Enormous achievements have been witnessed over the journey and currently the record efficiency of BHJ solar cells has reached over 10% (Figure 1). New materials, especially the donor materials in the blend, have been developed in an expanding rate, with new design strategies, new building blocks and new polymerization methods at the same time. For the acceptor part, fullerene based derivatives, PC61BM and PC71BM, are still the first choice for researchers. As the energy conversion process involves charge transfer over the donor/acceptor interface, the morphology of the donor/acceptor is therefore essential for an efficient power conversion process. The morphology of the thin film, however, is very sensitive to the processing conditions, such as the materials used, solvent, solvent additives, annealing, spincoating conditions, etc. A tedious but worthwhile optimization process of all these parameters has to be carried out to explore the full potential of any newly synthesized polymer donor material or any new acceptor material. So far, the choice of the best condition is still based on a trial-and-error approach. Furthermore, problems arise as the OPV technology is translated from the lab-scale to industrial scale, e.g., how to achieve the optimal morphology of a cm2 device in the industrial scale, how to optimally process the OPV module, and how to improve the device stability by suitable encapsulation. The solutions to these questions are by no means trivial. Most probably a rediscovery process has to be carried out to optimize all the parameters associated with the industrial scale production.

As the OPV technology has gradually become business relevant and quite a number of companies are currently active in the OPV market to cash the promises of OPV researchers, more creative breakthroughs are in urgent need to solve the intrinsic efficiency and stability issues of current OPV technology. Other than further development of more efficient light harvesting materials, some new concepts such as ternary solar cell [23, 130], and modification of the solar cell structures [22,24], e.g., inverted solar cells, tandem solar cells, or tandem organic/inorganic solar cells would pave new ways to improve the efficiency of the solar cell. Further development of encapsulant materials with lower water/oxygen transmission rate would help the solar cells survive longer under ambient conditions [29]. The continuous optimization effort on the industrial roll-to-roll printing techniques would help minimize the gap between the best efficiency data from lab devices and the large scale OPV modules. These developments have to be fast, as tremendous work is spent to optimize current inorganic PV technology as well (Figure 1). Furthermore, BHJ based solar cells also have to compete with other organic material based PV technology, such as dye-sensitized solar cells [152] and perovskite solar cells [153]. There are also issues regarding the marketing of OPV based technology. As it is envisaged that OPV will not be able to compete with inorganic PV technology in the mainstream energy production market in the coming 5 to 10 years [28], niche markets, such as portable electronics chargers, flexible PV and wearable PV, are therefore sought in the short term. The light weight and the flexibility of OPV technology would become advantageous to survive in the market.

To end up our discussion, we will emphasize the nature of OPV research and related materials development. Any science and technology development, if it is aimed at large scale application, it should be robust, reproducible, affordable and efficient in its claimed function. The materials used in the device should be accessed in an easy and cheap way and the production process should be cost-effective. And more importantly, the commercial products should have attractive features to survive in the market. There are still a lot of obstacles for OPV researchers to conquer, but more opportunities in the future.
