高效率背電極指叉式異質介面太陽能電池

Abstract

本實驗根據2014年德國Zielke, Dimitri團隊所提出的Backpedot(Back Junction)太陽能電池，我們加以改進製成指叉式Backpedot(Back Contact Back Junction)太陽能電池。從這幾年的PEDOT:PSS/Si異質介面太陽能電池觀察，大部分的接面都處於照光，容易造成部分太陽光被PEDOT:PSS所吸收而無法有效轉換成光電流，除此之外，正接面(Front junction)太陽能電池的背面幾乎無法有效地進行鈍化處理。為了解決這些缺點，PEDOT:PSS必須置於背面形成Back Junction，且須使用較薄的矽塊材，例如150um厚的矽晶片，才能有效地降低載子覆合的機率。在此實驗我們正負電極同時至於背面，使照光面沒有任何的金屬遮蔽效應，大大的提高了照光面積，也同時增加了短路電流(Jsc)。 在電極上，我們選擇指叉式電極，希望可以降低電子到相對應電極間的傳遞距離，且此種圖形的電極可以觀察出不同比例下的激發層對太陽能電池的影響。我們選擇57%、67%以及74%等三種不同PEDOT:PSS所佔的整體面積比，來選出最佳化的結果，而當PEDOT:PSS的面積比為67%時，得到了15.68%的效率，因為在此比例下，少數載子和多數載子的行走距離為三者中最佳化的結果。再者，在維持67%的面積比下，我們將原本的200um的鋁電極線寬，增加到300um，使正負電極間的距離由原本的200um縮減至150um，效率由原本的15.68%增加到17.59%，Fill Factor也從0.544增加到0.601，因為電子到達相對應的電極距離縮短了，所以Fill Factor也變好了。 由BCBJ太陽能電池結構可以看出背光面並沒有完全被電極所覆蓋，仍有裸露出少部分的矽表面，所以我們提出雙面太陽能電池的想法，在背光面的位置架設一面聚光鏡，使太陽能電池面積外的光源經由鏡面的反射至背光面，使電池兩面都可以照光。PEDOT:PSS面積比為67%，鋁電極線寬為300um的指叉式Backpedot太陽能電池背面照光所得到的效率為0.98%，再加上照光面照光後所得到的效率17.59%，當在1.5AM的環境下，兩面同時照光，則理想上我們會得到約18.6%的效率。

This thesis aims at the improvement of back-junction type PEDOT:PSS/Si hybrid solar cells, which was first proposed and demonstrated by Zieke and coworkers with a decent power conversion efficiency. The limited efficiency of current PEDOT:PSS/Si solar cells were basically due to the facts that the heterojunction was located on the front of the cell, resulting in a parasitic light absorption within the organic material. In addition, the rear surface of those front-junction solar cells was usually poorly passivated. To overcome above difficulties, one must place the PEDOT:PSS on the rear side of the device and the solar cell must be fabricated on a thin Si wafer to reduce carrier recombination. By adapting back contact cross-finger type PEDOT:PSS/Si heterostructures on thin Si wafers, we are able to reduce the shading loss to zero and enhance carrier collection. The absence of the metallization grid on the front side increases the short- circuit current (Jsc) of the cells dramatically. BackPEDOT cells with cross-finger type electrodes of different back emitter shading ratios were fabricated and tested. An optimized efficiency of 15.7% was achieved from cells with a shading ratio of 67%. It indicates that, at this particular shading ratio, diffusion lengths and carrier collection efficiency were optimized for both majority and minority carriers. By further increasing the width of Al electrodes from 200 um to 300 um and reducing the pitch between anode and cathode from 200 um to 150 um, the Fill Factor and efficiency of the BackPEDOT solar cells were further improved from 0.54 to 0.6 and from 15.7 % to 17.6%, respectively. Due to the cross-finger type electrodes employed on the devices, the rear side of the cells is not fully covered by the electrodes, which means that there are still exposed organic-silicon junction areas that can absorb sunlight. An efficiency of 1% was measured when the rear side of the solar cell was under 1.5AM illumination. Therefore, in principle, one can expect a total conversion efficiency of 18.6% when both sides of the cell are illuminated simultaneously.