**1. Introduction**

Polymer solar cells are of tremendous interests due to their attractive properties such as flexibility, ease of fabrication, low materials and energy budget. However, organic materials have short exciton diffusion length and poor charge mobility, which can greatly decrease the performance of polymer solar cells. These challenges can be effectively overcome through the use of the bulk heterojunction (HJ) structure because it can guarantee the effective exciton dissociation and carrier transport simultaneously if a proper bicontinuous interpenetrating network is formed in the active layer. Based on this structure, the performance of polymer solar cells has been improved steadily in the past decade.

The performance of a polymer solar cell is mainly determined by the short-circuit current density (*JSC*), the open circuit voltage (*VOC*), and the fill factor (*FF*), given that *η=JSCVOCFF/Pin* (where *η* is power conversion efficiency, *PCE*, and *Pin* is the incident optical power density). *VOC* has a direct relationship with the offset energies between the highest occupied molecular orbital of Donor (*D*) material and the lowest unoccupied molecular orbital of Acceptor (*A*) material (Cheyns et al., 2008). Since the *D* and *A* materials are intimately mixed together in the bulk HJ structure and their interfaces distribute everywhere in the active layer, it is difficult to increase *VOC* by changing *D*/*A* interface property for a given material system (such as poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester, P3HT:PCBM). Thus the usually used optimization method is to improve *JSC* and *FF*.

*JSC* greatly depends on the optical interference effect in polymer solar cells. Because of the very high optical absorption ability of organic materials, the active layer is very thin and typically from several ten to several hundred nanometers. This thickness is so thin compared to the incident light wavelength that the optical interference effect has to be carefully considered. Depending on the thicknesses and optical constants of the materials, the optical interference causes distinct distributions of the electric field and energy absorption density. Due to this effect, *JSC* shows an obvious oscillatory behavior with the variation of active layer thickness. In order to gain a high *PCE*, the active layer thickness needs to be well optimized according to the optical interference.

Effects of Optical Interference and Annealing on the

with the increase of the active layer thickness.

**2.1 Theory** 

refraction *nni*

be calculated by

refraction,

point *z*. *Q z*(, )

where *h* is Planck constant, and

**2.1.1 Exciton generation** 

where *c* is the vacuum speed of light, <sup>0</sup>

the absorption coefficient,

be considered at the same time.

Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells 3

dissociation probability is not 1 and depends on some factors such as electric field and temperature. When the active layer thickness is increased to optimize the light absorption, the electric field in the blend layer decreases, which lowers down the exciton-to-freecarrier probability and makes charge collection less effective simultaneously. As a result, *JSC* may become low, although the thickness has been optimized for better light absorption. Thus to obtain a higher *JSC*, both the optical and the electric properties should

Some previous works (Lacic et al., 2005; Monestier et al., 2007) studied the characteristic of *JSC*. However, they neglected the influence of exciton-to-free-carrier probability, which is important for polymer solar cells. Another study (Koster et al., 2005) considered this factor, but they neglected the optical interference effect, which is a basic property for the very thin organic film. All the above studies are based on the numerical method, and it is not easy to solve the equations and understand the direct influence of various parameters on *JSC*. In this part, a model predicting *JSC* is presented by using very simple analytical equations. Based on this model, the effects of optical interference on *JSC* is investigated. Besides, the carrier lifetime is also found to be an important factor. By considering the optical interference effect and the the carrier lifetime, it is found that when the lifetimes of both electrons and holes are long enough, the exciton-to-free-carrier dissociation probability plays a very important role for a thick active layer and *JSC* behaves wavelike with the variation of the active layer thickness; when the lifetime of one type of carrier is too short, the accumulation of charges appears near the electrode and *JSC* increases at the initial stage and then decreases rapidly

The active layer in polymer solar cells absorbs the light energy when it is propagating through this layer. How much energy can be absorbed depends on the complex index of

> 0 <sup>1</sup> (, ) () <sup>2</sup> *Qz c nEz*

 

(, ) (, ) (, ) *Q z G z Q z h hc* 

800 300 *Gz Gz d* () (, )

average of the energy dissipated per second for a given wavelength

exciton, the exciton generation rate at position *z* in the material is given by

generated by the material at position *z* in solar spectrum are calculated by

 4 / 

of the materials. At the position z in the organic film (Fig. 1 (a)), the time

2

have the unit of <sup>3</sup> *W m*/ . Assuming that every photon generates one

  , and *E(z)* the electrical optical field at

*<sup>j</sup>* (1)

the permittivity of vacuum, *n* the real index of

(2)

(3)

is the frequency of incident light. The total excitons

of incident light can

Besides the serious optical interference effect, *JSC* also suffers from the non-ideal free carrier generation, low mobility and short carrier lifetime. In order to reduce the exciton loss and guarantee the efficient carrier transport, the optimal interpenetrating network, or to say, the optimal morphology is desired in the bulk HJ structure. In order to achieve an optimal morphology, a thermal treatment is usually utilized in the device fabrication, especially for the widely used P3HT:PCBM solar devices. It is found that the sequence of the thermal treatment is critical for the device performance (Zhang et al., 2011). The polymer solar cells with the cathode confinement in the thermal treatment (post-annealed) show better performance than the solar cells without the cathode confinement in the thermal treatment (pre-annealed). The functions of the cathode confinement are investigated in this chapter by using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), optical absorption analysis, and X-ray diffraction (XRD) analysis. It is found that the cathode confinement in the thermal treatment strengthens the contact between the active layer and the cathode by forming Al–O–C bonds and P3HT-Al complexes. The improved contact effectively improves the device charge collection ability. More importantly, it is found that the cathode confinement in the thermal treatment greatly improves the active layer morphology. The capped cathode effectively prevents the overgrowth of the PCBM molecules and, at the same time, increases the crystallization of P3HT during the thermal treatment. Thus, a better bicontinuous interpenetrating network is formed, which greatly reduces the exciton loss and improves the charge transport capability. Meanwhile, the enhanced crystallites of P3HT improve the absorption property of the active layer. All these aforementioned effects together can lead to the great performance improvement of polymer solar cells. Besides the thermal treatment sequence, temperature is another very important parameter in the annealing process. Various annealing temperatures have also been tested to find the optimized annealing condition in this chapter.

The contents of this chapter are arranged as the following: Section 2 introduces the effects of the optical interference on *JSC* in polymer solar cells by considering the non-ideal free carrier generation, low mobility and short carrier lifetime at the same time; Section 3 investigates the influence of the sequence of the thermal treatment on the device performance with emphasis on the cathode confinement in the thermal treatment; based on the optical interference study and the proper thermal treatment sequence, the overall device optimization is presented in Section 4. At last, a short conclusion is given in Section 5.
