IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 1, JANUARY

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1 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 1, JANUARY Performance Comparison of Conventional and Inverted Organic Bulk Heterojunction Solar Cells From Optical and Electrical Aspects Dazheng Chen, Chunfu Zhang, Member, IEEE, Zhizhe Wang, Jincheng Zhang, Qian Feng, Shengrui Xu, Xiaowei Zhou, and Yue Hao, Senior Member, IEEE Abstract The conventional and inverted organic solar cells (OSC and IOSC) based on the bulk heterojunction structure are investigated from both optical and electrical aspects. When the optical aspect is considered only, with the increase of the active layer thickness, the number of photons absorbed in the active layer and the external quantum efficiency tend to increase with the obvious interference behavior for both OSC and IOSC. However, compared to OSC, IOSC shows a better performance except for the thicknesses around which the interference maxima of OSC are obtained. When the electrical aspect is also considered, an effective area in the active layer will be induced by the charge drift length (L), and only the photons absorbed in this effective area have contribution to the photocurrent. By considering optical and electrical aspects together, OSC and IOSC show different behaviors. Compared to IOSC, OSC performs better for relatively thick active layers. Simultaneously, the optical modulation effect is also investigated by introducing an optical spacer layer. It is found that the optical spacer layer can notably enhance the performance of OSC with thin and thick active layers, while it could only degrade the performance of IOSC with relatively thick active layers. Index Terms Conventional structure, electrical aspect, inverted structure, optical aspect, organic solar cells. I. INTRODUCTION LOW IN COST, light in weight, and flexible in mechanics, the solution-processed organic solar cells have aroused worldwide interest and have been the promising substitute for the traditional silicon-based solar cells [1], [2]. However, they are still not available for commercialization since their power conversion efficiency (PCE) is relatively low [3]. Therefore, many research works have focused on employing new materials and device structures to improve the PCE of organic solar cells. The milestone is the introduction of the bulk heterojunction structure consisting of an interpenetrating network of electron donor and acceptor materials [4]. Based on this idea, the conventional organic solar cell (OSC) with poly (3-hexylthiophene-2,5-diyl):[6, 6]-phenyl C 61 butyric acid methyl ester (P3HT:PCBM) blend shows a superior performance [5], and the PCE over 6% has been reported [6]. Recently, the inverted organic solar cell (IOSC) has been Manuscript received May 16, 2012; revised September 24, 2012; accepted September 26, Date of publication November 12, 2012; date of current version December 19, This work was supported in part by the NSFC under Grant and in part by the Fundamental Research Funds for the Central Universities under Grant K The review of this paper was arranged by Editor A. G. Aberle. The authors are with the Xidian University, Xi an , China ( zhangchunfu@126.com; yhao@xidian.edu.cn). Digital Object Identifier /TED introduced as the possible candidate for OSC to remedy the drawback of low air stability which results from the low-workfunction metal electrode and the highly acidic PEDOT:PSS interfacial layer in OSC [7], [8]. In addition, it has been reported that the PCBM possesses higher concentration on the ITO side and the P3HT possesses higher concentration on the metal electrode side in the active layer, which is more favorable for the charge transport in IOSC [9]. Nowadays, both OSC and IOSC are attracting research interest. In general, organic photovoltaic devices generate power through three major processes: exciton generation (absorption), exciton harvesting (exciton migration and dissociation), and charge transport [10]. The first process is the optical mechanism, and the other two processes constitute the electrical aspect. Therefore, to achieve better performances of OSC and IOSC, most of the efforts are concentrated on these two important aspects: optical aspect (such as the transmittance, optical modulation effect, and optical electric field distribution [11], [12]) and electrical aspect (such as the resistivity of buffer layer and the energy alignment [13], [14]). However, most of the previous works are only done for OSC or IOSC separately, and there are few reports about the systemic comparison of OSC and IOSC for their different performances besides the air stability. As a result, this paper aims to investigate the difference between OSC and IOSC from both optical and electrical aspects. II. METHOD In this paper, we assume that one absorbed photon produces one exciton in the active layer and one exciton divides into two free charges (one electron and one hole), and one electron (or hole) is collected by the cathode (or anode). As a result, the number of photons absorbed in the active layer can be used as a substitute for the maximum possible short circuit current density. Thus, the external quantum efficiency (EQE) can be simplified as the ratio of the number of photons absorbed in the active layer to the number of incident photons. To calculate the number of absorbed photons (or excitons) in the active layer, the energy flow dissipation per second at single wavelength in the active layer, Q is given as Q = 1 2 cε 0αn E 2 (1) where c is the speed of light in vacuum, ε 0 is the permittivity of vacuum, α is the absorption coefficient, n is the real /$ IEEE

2 452 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 1, JANUARY 2013 Fig. 1. Schematic illustrations of (a) OSC and (b) IOSC. index of refraction, and E 2 is the total optical electric field intensity in the multiplayer stack at single wavelength [15]. Then, the number of photons absorbed in the active layer can be expressed as N = 800 nm λ=300 nm Q(λ) λ dλ (2) hc where N represents the number of absorbed photons, hc/λ is the photon energy at a specified wavelength λ, h is the Planck constant, and c the speed of light in vacuum. In the simulations of this study, OSC has the structure of glass/ito(150 nm)/pedot:pss(50 nm)/p3ht:pcbm(x nm)/ Al (100 nm), and IOSC has the structure of glass/ito(150 nm)/ TiO x (10 nm)/p3ht:pcbm(x nm)/moo 3 (10 nm)/al(100 nm) as shown in Fig. 1. To investigate the difference between OSC and IOSC from the optical aspect, the number of photons absorbed in the active layer and the EQE are obtained from transfer matrix method by calculating the interference of coherent reflected and transmitted waves at each interface in the stack. The calculations require the knowledge of the wavelengthdependent complex index of refraction of each material [16], [17]. Here, the optical constants of each material are obtained from the literatures [17] [19], the devices are illuminated with AM 1.5G solar spectra from 300 to 800 nm, and the detailed optical calculation processes are not presented here since the transfer matrix method is widely applied [16] [18]. Furthermore, aside from the optical factors, the electrical aspect is further considered by introducing the concept of charge drift length (L). Then, the performance comparison of OSC and IOSC based on optical electric field distributions is discussed in the next section. III. RESULTS AND DISCUSSIONS A. Considering Only Optical Factors In this part, the number of photons absorbed in the active layer as a function of the active layer thickness is obtained for OSC and IOSC, as well as the EQE as a function of wavelength and the active layer thickness. Furthermore, the optical modulation effect is investigated by inserting a ZnO layer between the layers of P3HT:PCBM and Al for OSC and changing the thickness of the MoO 3 layer for IOSC, respectively. Fig. 2. Number of photons absorbed in the active layer versus the active layer thickness with various thicknesses of the optical spacer layer. (a) OSC with ZnO layer thicknesses ranging from 0 to 30 nm. (b) IOSC with MoO 3 layer thicknesses ranging from 10 to 40 nm. (c) Comparison of OSC (without ZnO) and IOSC (with 10-nm MoO 3 ). Comparison of Photons Absorbed in the Active Layer: For OSC in Fig. 2(a), it is clear that the number of absorbed photons increases with the active layer thickness and a behavior of oscillation can be observed. The same effect also exists in IOSC. As shown in Fig. 2(b), the increasing thickness of the MoO 3 layer leads to the shift of interference maxima toward lower thicknesses and the increase of absorbed photons, particularly for the relatively thin active layers. Therefore, the MoO 3 layer may also act as an optical spacer layer, and this effect is very similar to that of the ZnO optical spacer layer for OSC. Thus, for the typical OSC and IOSC devices, the influence of the active layer thickness on the number of absorbed photons is similar, as well as the optical modulation effect, and its less impact on IOSC is observed when the active layers are relatively thick (about 150 nm). Considering OSC without the ZnO layer and IOSC with a 10-nm MoO 3 layer shown in Fig. 2(c), it is clear that the number of absorbed photons in IOSC is larger than that in OSC at any active layer thickness except for the thicknesses around which the interference maxima of OSC are obtained. In other words, for most of the active layer thicknesses, the light absorption in IOSC is more efficient, hence the larger contribution to the photocurrent. It has been reported that the light loss induced by the reflection at the ITO/TiO x interface and that at the ITO/PEDOT:PSS interface are nearly equivalent [18]. However, the light absorption loss in the TiO x layer is smaller than that in the PEDOT:PSS layer [20]. In addition, the MoO 3 layer in IOSC can act as an optical spacer layer. As a result, one can say that the better light absorption in IOSC is attributed to the nearly equal amount of entering light and

3 CHEN et al.: COMPARISON OF CONVENTIONAL AND INVERTED ORGANIC SOLAR CELLS 453 Fig. 3. Comparison of EQE as a function of the active layer thickness for OSC and IOSC with six different incident light wavelengths. smaller absorption loss in the TiO x layer, as well as the optical modulation effect caused by the MoO 3 layer. Although IOSC shows a better performance than OSC at most of the active layer thicknesses, it is noted that the performance of IOSC is slightly lower than that of OSC around the interference peaks as shown in Fig. 2(c). One possible reason is the parasitic absorption in the MoO 3 layer because not only the optical modulation effect but also the absorption loss could be caused by the MoO 3 layer, which produces a tradeoff [20]. Comparison of EQE: For the EQE of OSC and IOSC shown in Fig. 3, a remarkable increase with the active layer thickness and an oscillation behavior can be observed. A conclusion can be obtained from Fig. 3 that IOSC performs better than OSC at most of the active layer thicknesses for the light ranging from 400 to 650 nm. However, at the active layer thicknesses where the interference maxima of OSC are obtained, the EQE of OSC is close to or even higher than that of IOSC. To well investigate the difference between OSC and IOSC, the values of the first maxima of the EQE and the corresponding active layer thicknesses at different wavelengths are presented in Table I. The clear oscillation behavior at the corresponding active layer thicknesses of the maximum EQE can be observed for both OSC and IOSC, and OSC exhibits a slighter thickness oscillation behavior than IOSC. It is evident that the slight oscillation of maxima at single wavelength is beneficial to the final maxima in the range including all wavelengths. Therefore, the results from Table I can also be used to explain the results shown in Fig. 2(c). In addition, we also investigate the EQE as a function of wavelength at four different active layer thicknesses. Compared to OSC, it is found that IOSC shows higher EQE in the main absorption range (from 400 to 650 nm) for both thin and thick TABLE I VALUES OF FIRST INTERFERENCE MAXIMA OF EQE AND CORRESPONDING ACTIVE LAYER THICKNESSES (x) AT DIFFERENT WAVELENGTHS (λ) (40 and 160 nm) active layers except for the thicknesses (80 and 220 nm) around which the interference maxima of OSC are obtained. This result agrees well with the previous discussions. B. Considering Electrical Factors (Charge Drift Length) The performance comparison of OSC and IOSC only from the optical aspect has been presented in the aforementioned section. In the following, the electrical factors are also included in the discussions by using the concept of charge drift length (L). For a supposed value of L, its influence on the distributions of optical electric field at different active layer thicknesses and the optical modulation effect are discussed in this part. Charge Drift Length (L): In the process of charge transport, it is believed that not all generated free charges in the active layer can be collected due to the limited mobility and lifetime of the charges (electrons or holes), a fact determined by the properties of employed materials in the devices. The comprehensive effect of mobility and lifetime can be considered by using a concept of charge drift length (L). TheL is simply defined as the maximum length that a charge carrier can drift

4 454 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 1, JANUARY 2013 within its lifetime and can be expressed as L = μτe, where L represents the charge drift length, μ is the mobility, τ is the lifetime of charge carriers, and E is the electric field. Considering the electroneutrality, the number of electrons collected by the cathode is equivalent to the number of holes collected by the anode. Therefore, eliminating other electrical losses for an ideal case, the number of low-mobility charge carriers and their L determine the final number of collected charges. Absolutely, if the hole and electron have similar mobilities, the transportations of holes and electrons in the active layer are balanced, which will result in high cell performance. In this way, the performance difference between OSC and IOSC may not be obvious because of the similar situation of L. Therefore, to investigate the influence of L, we should consider L from two aspects: materials and device structures. On the one hand, for the common used active layer materials in bulk heterojunction cells, the electron and hole usually have different mobilities. On the other hand, organic solar cells are mainly based on the conventional and inverted structures, but the direction of charge transport is opposite in these two structures. Therefore, the cases of L are not the same for different materials and device structures. In this paper, both OSC and IOSC employ the blend of P3HT:PCBM, and holes have lower mobility than electrons in this material. Therefore, holes play a dominant role in the charge transport process by considering L. To make the discussion convenient, the L of the holes is supposed to be 100 nm. Although the real L may be different from this value, the achieved conclusions also hold in the qualitative analysis for all the organic solar cells. Here, for the devices with the active layer thickness less than 100 nm, L does not work. However, when the thickness of the active layer is larger than 100 nm, L must be taken into account, and it will induce an effective area in the active layer. Moreover, the absorbed photons out of this effective area will be lost and have no contribution to the photocurrent. It should be noted that the position of the effective area is also different for OSC and IOSC since the direction of the charge transport is opposite, and interesting results may be obtained from these two structures. The effect of L on the performances of OSC and IOSC, as well as the optical modulation effect, will be discussed as follows. Effect of L on Device Performance: Considering L, the difference between OSC and IOSC is investigated by analyzing the optical electric field distributions of OSC and IOSC with different active layer thicknesses. The incident light wavelengths are chosen to be 400, 500, and 600 nm. Correspondingly, the number of absorbed photons and the EQE are presented in Table II. Here, the active layer thicknesses are chosen to be 45, 150, and 230 nm to represent the thin and thick devices, and the results are discussed as follows. First, consider the devices with the 45-nm active layer. Obviously, the influence of L can be neglected since the active layer is thinner than 100 nm. For both OSC and IOSC, it is found that the optical spacer layer (the ZnO and the MoO 3 layer) results in an enhanced light absorption induced by the redistribution of the optical electric field. The corresponding optical electric field distributions are not shown here since the similar discussions can be seen in many reports [11], [12]. TABLE II NUMBER (N/10 18 m 2 s 1 ) OF PHOTONS ABSORBED IN THE EFFECTIVE AREA AND EQE OF OSC AND IOSC WITH (a) 45, (b) 150, AND (c) 230 nm ACTIVE LAYERS AT THE LIGHT WAVELENGTHS (λ) OF 400, 500, AND 600 nm Simultaneously, the enhanced performance of OSC and IOSC can also be observed from Table II(a), and the IOSC with a 10-nm MoO 3 layer performs better than the OSC without a ZnO layer in this case. Second, consider the devices with the 150-nm active layer. At this moment, L begins to work since the active layer thickness is larger than 100 nm. As shown in Fig. 4(a) and (c), only the lower part of the active layer with the 100-nm thickness (shadow area) is effective for OSC, and for IOSC, only the upper part with the 100-nm thickness (shadow area) works as shown in Fig. 4(b) and (d). For both OSC and IOSC, Fig. 4 illustrates that the positions of interference maxima tend to shift outside the effective area (shadow area) after inserting a 20-nm ZnO layer and increasing the thickness of the MoO 3 layer to 20 nm. Simultaneously, the decrease of the area below the curves also has a negative effect on the device performance, which can be quantified in Table II(b) that the insertion of an optical spacer layer leads to the decrease of the number of photons absorbed in the effective area and the EQE. It is apparent that, although the optical spacer layer (20-nm ZnO for OSC and MoO 3 for IOSC) has nearly no effect on the performance improvement for both OSC and IOSC with 150-nm active layers, the IOSC with a 10-nm MoO 3 layer still has a better performance than the OSC without a ZnO layer. In addition, the degeneration of the number of photons absorbed in the effective area and the EQE can be seen at larger thickness by comparing Table II(a) and (b). Further Discussion of Thick Active Layer Devices: From Fig. 4(c), at the wavelength of 400 nm, an interesting phenomenon should be noted that the previous interference maximum in the effective area is replaced by a new larger maximum after inserting a ZnO layer, which leads to a larger area below the curve in the effective area. As a result, with the departure of the initial maximum, a shift of the new maximum into the effective area may be obtained by increasing the thickness of the optical spacer layer. At the same time, the fact whether IOSC has a better performance for thicker devices should also be identified.

5 CHEN et al.: COMPARISON OF CONVENTIONAL AND INVERTED ORGANIC SOLAR CELLS 455 Fig. 4. Normalized optical electric field distributions for OSC and IOSC with 150-nm active layers. (a) and (c) represent the OSC without and with a 20-nm ZnO layer. (b) and (d) represent the IOSC with the 10- and 20-nm MoO 3 layers. Specifying the wavelength as 500 nm, (e) and (f) represent the optical electric field of OSC and IOSC with optical spacer layer (ZnO and MoO 3 ) thicknesses ranging from 40 to 140 nm. The L is supposed to be 100 nm, and the shadow area represents the effective area in the active layer. Increasing the thickness of the ZnO layer from 40 to 140 nm for OSC, the distributions of optical electric field at specified 500 nm are displayed in Fig. 4(e). It can be observed that one interference maximum drifts gradually into the effective area with the departure of the initial maximum and departs the effective area when the thickness of the ZnO layer exceeds 120 nm. From Table III(a), compared to the device without the ZnO layer, the number of photons absorbed in the effective area and the EQE could be enhanced by the optical modulation effect at most of the ZnO layer thicknesses. The results presented in Fig. 4(e) illustrate that the previous prediction of adding a new maximum in the effective area by increasing the optical spacer layer thickness is available for OSC. When it comes to IOSC, as shown in Fig. 4(f), the position of a new maximum drifts into the effective area for the thickness of the MoO 3 layer larger than 80 nm and resides in the effective area for the 140-nm MoO 3 layer. However, a significant difference between OSC and IOSC is implied in Table III(b). The IOSC with a 10-nm MoO 3 layer possesses more photons absorbed in the effective area and higher EQE than the cases with thicker MoO 3 layers, namely, the thicker optical spacer layers cannot enhance the device performance. The difference between OSC and IOSC may be explained from two aspects. On the one hand, the maximum required to shift is close to the effective area for OSC, and the curve must pass an entire region of minimum before the new maximum comes into TABLE III NUMBER (N/10 18 m 2 s 1 ) OF PHOTONS ABSORBED IN THE EFFECTIVE AREA AND EQE OF DEVICES WITH 150-nm ACTIVE LAYER AT DIFFERENT OPTICAL SPACER LAYER THICKNESSES [(a) FOR OSC AND (b) FOR IOSC] the effective area for IOSC. On the other hand, the amplitude of the maximum and the area below the curves possess more decrease with the shift of the maximum into the effective area for IOSC. Therefore, a thicker optical spacer layer is needed to shift the position of the maximum, and less absorbed photons and lower EQE are obtained for IOSC. In addition, when the active layer thickness increases to 230 nm, it can be observed from Table II(c) that the optical modulation effect cannot

6 456 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 1, JANUARY 2013 the fact that the minima of optical electric field distributions just reside in the effective area as shown in Fig. 4(a) and (c). At the same time, the less photons absorbed in IOSC with thicker active layers can attribute to the smaller amplitudes of maxima. Then, the prediction of a better performance of IOSC with thicker active layers is not valid, and the reason is also given in Fig. 5(c). On the base of Fig. 5, the difference between OSC and IOSC discussed in Fig. 4 and Table III is also explained. In addition, considering the all-polymer organic solar cells in which electrons have lower mobility, the L of electrons plays a dominant role; thus, the positions of the effective area for the same-type devices (OSC or IOSC) are opposite, and the conclusions from the similar discussions are different compared to those of the P3HT:PCBM devices. Fig. 5. Number of photons absorbed in the effective area versus the active layer thickness with various thicknesses of the optical spacer layer. (a) OSC with ZnO layer thicknesses ranging from 0 to 50 nm. (b) IOSC with MoO 3 layer thicknesses ranging from 1 to 40 nm. (c) Comparison of OSC (without ZnO) and IOSC (with 10-nm MoO 3 ). improve the performances of OSC and IOSC, and IOSC is no longer superior to OSC. Therefore, to understand the difference between OSC and IOSC by considering L, the influence of the active layer thickness on the device performance should be investigated in detail. Considering the influence of L, the number of photons absorbed in the effective area as a function of the active layer thickness with various optical spacer layer thicknesses for OSC and IOSC is investigated. For OSC, as shown in Fig. 5(a), the thickness of the ZnO layer is chosen from 0 to 50 nm, and the curves below 100 nm are identical to the case in Fig. 2(a) as L is supposed to be 100 nm. The optical modulation effect can notably improve the performance of the devices with thick (near 180 nm) active layers, and the oscillation behavior of the number of photons absorbed in the effective area is agreed to the corresponding changes in Table II. For the IOSC described in Fig. 5(b), the optical modulation effect cannot improve the number of photons absorbed in the effective area any more for thick devices, and no behavior of oscillation can be observed. The comparison of OSC and IOSC taking L into account is displayed in Fig. 5(c), wherein OSC has no ZnO layer inserted and IOSC has a 10-nm MoO 3 layer. Considering L, itisworth to note that OSC shows a better performance when the active layer thickness is larger than about 180 nm, which distinctly differs to the results discussed only from the optical aspect. The difference of the top and bottom curves beyond 100 nm in Fig. 5(c) is caused by the great loss of photons in the active layer, which is also implied in Fig. 4. For the bottom curves, the less photons of OSC below about 180-nm active layer is due to IV. CONCLUSION From the optical aspect, OSC and IOSC based on P3HT:PCBM have the same tendency in the number of photons absorbed in the active layer and the EQE as well as the similar influence of the optical modulation effect. However, for the typical structures of OSC and IOSC discussed in this study, IOSC performs better than OSC except for the case wherein the interference maxima of OSC are obtained, which is due to the better light absorption in IOSC and the possible absorption loss caused by the MoO 3 layer. Furthermore, if L is considered, a great deal of photons beyond the effective area will be lost when the active layer thickness is larger than L. For the common P3HT:PCBM cells, when considering the effect of L, it should be noted that OSC has a better performance than IOSC except for the relatively thin devices. 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