• No results found

In this chapter, a model for the open-circuit voltage was introduced. By studying the dependency of Voc on incident light intensity, it has been demonstrated that the Vocof BHJ solar cells is inconsistent with p-n junction models: Whereas the experimental data

showed that the slope S of Vocas a function of ln(I)is equal to Vt, the p-n junction model predicts a slope of nVt, where n ranges from 1.35 to 1.98. This phenomenon was observed for two different PPV derivatives as donor material. The main cause of this discrepancy lies in the fact that the strong voltage dependence of the photogenerated current is not taken into account.

An alternative model for the open-circuit voltage has been presented, based on the notion that the quasi-Fermi potentials are constant throughout the device. Subsequently, a formula for Vocwas derived, consistently explaining the light intensity dependence of the open-circuit voltage of polymer/fullerene bulk heterojunction devices with Ohmic contacts. Next, the predictions of the MIM model and its relation to other types of solar cells have been discussed.


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Short-circuit current of bulk heterojunction solar cells


A typical feature of polymer/fullerene based solar cells is that the short-circuit current density does not scale exactly linearly with light intensity. Instead, a power law relationship is found given by JscIα, where α ranges from 0.85 to 1. In a number of reports this deviation from unity is speculated to arise from the occurrence of bimolecular recombination. In this chapter, simple analytical models are discussed, showing that the short-circuit current should show a power law behavior on intensity, JscIα, with 0.75 < α < 1, depending on the mobility of both carriers. By applying the numerical model as outlined in chapter 2, it is demonstrated that the experimentally observed intensity dependence is indeed caused by space charge effects and does not originate from bimolecular recombination losses. This explanation is verified for an experimental model system with a mobility difference that can be tuned from one to three orders of magnitude by changing the post-production annealing treatment.

4.1 Introduction

One of the key parameters of any solar cell is the short-circuit current density Jsc, and its optimization is of great importance for the further improvement of organic photovoltaic devices. What determines the short-circuit current? An important issue, in this respect, is the dependence of Jsc on incident light intensity I. Several authors have reported a power law dependence of Jsc, i.e. JscIα, where α ranges typically from 0.85 to 1 for polymer/fullerene based solar cells.[1–7] Thus far, the deviation from α = 1 has been conjectured to arise from a small loss of carriers via bimolecular recombination.[2,4,7]As a first step, this section deals with three simple analytical expressions for the photocur-rent generated by a BHJ solar cell. The concept of space-charge-limited photocurphotocur-rent, introduced in this section, will prove to be of use in explaining the intensity dependence of Jsc.

4.1.1 Uniform field approximation

Consider the simple case of a photoconductor with non-injecting contacts and a uniform electric field distribution. Goodman and Rose derived that, under the assumption of negligible recombination of charge carriers,[8]

Jph =qGL, (4.1)

meaning that in this case all photogenerated charge carriers are simply extracted and the current density depends only on the generation rate G. In their derivation, Goodman and Rose took only drift of charge carriers into account and neglected the contribution of diffusion. Sokel and Hughes carried this analysis one step further by including diffusion of carriers, finding[9]

Jph =qGLhexp(V/Vt) +1

exp(V/Vt) −12VVti, (4.2) where V is the voltage drop across the active layer. For a BHJ device this voltage drop is given by V0Va. The result by Sokel and Hughes shows two regimes: A linear dependence of Jphon voltage for small biases, while reducing to Eq. (4.1) at moderately high bias (including short-circuit conditions), see Fig. 4.1.

On the basis of Eqs. (4.1) and (4.2) one expects that Jsc = qGL and, hence, that Jsc

is proportional to the incident light intensity. From experiments it is evident that this is not always true; for some systems Jsc is clearly sublinear in light intensity. The fact that Eqs. (4.1) and (4.2) do not depend on charge carrier mobility is a consequence of the assumption of no recombination, thereby ensuring that all carriers exit the device and that the electric field is uniform. In this chapter, we shall see that it is exactly this assumption of a uniform electric field that fails when Jscshows sublinear behavior.

0.01 0.1 1

Figure 4.1: The photocurrent data (symbols) of an MDMO-PPV/PCBM device, as discussed in subsection 2.4.2, together with the Sokel and Hughes result Eq. (4.2) (solid line) and the prediction by Goodman and Rose [dashed line, Eq. (4.1)]. At low effective applied voltages JphV0Va

(regime I), while at V0Va'0.3 V, the photocurrent saturates to qGL (regime II).

4.1.2 Space-charge-limited photocurrents

Mihailetchi et al. have demonstrated that a large difference in electron- and hole mobility, accompanied by a low mobility of the slowest carrier, may lead to space-charge-limited photocurrents.[10] The extraction of photogenerated carriers is governed by the mean carrier drift length w, which is the mean distance a carrier travels before recombination occurs. When both the electron (wn) and hole (wp) drift lengths are larger than the active layer thickness, then the charges will readily flow out without distorting the field in the device, see Fig. 4.2(a). However, in case wnwp and wp < L there will be a net positive space charge near the anode, see Fig. 4.2(b). There exist three regimes in the device: Near the cathode the electron density is much larger than the hole density;

this is a small region (I). Next to this region, there exists a balance between electron and hole density, yielding a neutral region (II). Near the anode, the holes dominate the device (III), resulting in a large net space charge and concomitant large voltage drop, as indicated in Fig. 4.2(b). The large field strength in region III facilitates the extraction of holes, ensuring that the extraction current of holes and electrons is equal.

When the photocurrent is space-charge-limited, the following relation holds:[8,10]

JphG0.75pV0Va. (4.3)

Thus, fully space-charge-limited photocurrents are characterized by a square-root de-pendence on voltage and are proportional to I0.75, irrespective of the amount of bimolec-ular recombination. On the other hand, non space-charge-limited devices have a linear dependence of Jphon I. Therefore, it is expected that the three-quarter power intensity dependence gradually increases to a linear dependence if the difference between the

mo-Figure 4.2: (a) Band diagram of a BHJ solar cell with balanced electron and hole mobilities; both types of charge carriers can readily flow out of the device and the field in the device is uniform.

(b) Band diagram in the case of hole accumulation and concomitant space-charge-limited behavior.

Near to the cathode, a small region dominated by electrons (I) exists next to a large neutral region (II), where electron and hole densities are comparable. Most of the potential drops across the hole accumulation layer (III) in order to facilitate the extraction of holes.

bility of electrons and holes is reduced. Since the occurrence of space charge is sensitive to the mobility difference, it is easy to understand that various material combinations will give different exponents α describing the intensity dependence of Jsc.

The experimental observation of space-charge-limited photocurrents in BHJ solar cells makes a very strong point in favor of the MIM model and cannot be explained by a p-n junction model. The fact that Eq. (4.3) describes the experimental data so well, even though Goodman and Rose originally derived it for one material instead of a blend, shows that the assumption of an effective medium holds.